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Physics Work Problems for High Schools

In this tutorial, we want to practice some problems on work in physics. All these questions are easy and helpful for your high school homework.

Work Problems: Constant Force

Problem (1): A constant force of 1200 N is required to push a car along a straight line. A person displaces the car by 45 m. How much work is done by the person?

Solution : If a constant force $F$ acts on an object over a distance of $d$, and $F$ is parallel to $d$, then the work done by force $F$ is the product of the force times distance.

In this case, a force of $1200\,{\rm N}$ displaces the car $45\,{\rm m}$. The pushing force is parallel to the displacement. So, the work done by the person is equal to \[W=Fd=1200\times 45=54000\,{\rm J}\] The SI unit of work is the joule, ${\rm J}$.

Problem (2): You lift a book of mass $2\,{\rm kg}$ at constant speed straight upward a distance of $2\,{\rm m}$. How much work is done during this lifting by you?

Solution : The force you apply to lift the book must be balanced with the book's weight. So, the exerted force on the book is \[F=mg=2\times 10=20\quad{\rm N}\] The book is lifted 2 meters vertically. The force and displacement are both parallel to each other, so the work done by the person is the product of them. \[W=Fd=20\times 2=40\quad {\rm J}\]

Problem (3): A force of $F=20\,{\rm N}$ at an angle of $37^\circ$ is applied to a 3-kg object initially at rest. The object has displaced a distance of $25\,{\rm m}$ over a frictionless horizontal table. Determine the work done by (a) The applied force (b) The normal force exerted by the table (c) The force of gravity

Solution : In this problem, the force makes an angle with the displacement. In such cases, we should use the work formula $W=Fd\cos\theta$ where $\theta$ is the angle between force $F$ and displacement. To this object, an external force $F$, normal force $F_N$, and a gravity force $w=mg$ are applied.

(a) Using vector decompositions, the component of the force parallel to the displacement is found to be $F_{\parallel}=F\cos \theta$. Thus, the product of this component parallel to the displacement times the magnitude of displacement gives us the work done by external force $F$ as below \begin{align*} W_F&=\underbrace{F\cos\theta}_{F_{\parallel}}d\\\\ &=(20\times \cos 37^\circ)(25)\\\\&=400\quad {\rm J}\end{align*} (b) Now, we want to find the work done by the normal force. But let's define what the normal force is.

In physics, ''normal'' means perpendicular. When an object is in contact with a surface, a contact force is exerted on the object. The component of the contact force perpendicular to the surface is called the normal force.

Thus, by definition, the normal force is always perpendicular to the displacement. So, the angle between $F_N$ and displacement $d$ is $90^\circ$. Hence, the work done by the normal force is determined to be \[W_N=F_N d\cos\theta=(30)(25)\cos 90^\circ=0\] (c) The weight of the object is the same as the force of gravity. This force applies to the object vertically downward, and the displacement of the object is horizontal. So, again, the angle between these two vectors is $\theta=90^\circ$. Hence, the work done by the force of gravity is zero.

Problem (4): A person pulls a crate using a force of $56\,{\rm N}$ which makes an angle of $25^\circ$ with the horizontal. The floor is frictionless. How much work does he do in pulling the crate over a horizontal distance of $200\,{\rm m}$?

Solution : The component of the external force parallel to the displacement does work on an object over a distance of $d$. In all work problems in physics, this force component parallel to the displacement is found by the formula $F_{\parallel}=F\cos \theta$. Thus, the work done by this force is computed as below \begin{align*} W&=F_{\parallel}d\\&=(F\cos\theta)d\\&=(56\cos 25^\circ)(200) \\&=10080\quad {\rm J}\end{align*} We could use the work formula from the beginning $W=Fd\cos\theta$ where $\theta$ is the angle between $F$ and $d$.

Problem (5): A worker pushes a cart with a force of $45\,{\rm N}$ directed at an angle of $32^\circ$ below the horizontal. The cart moves at a constant speed. (a) Find the work done by the worker as the cart moves a straight distance of $50\,{\rm m}$. (b) What is the net work done on the cart?

Solution :(a) All information to find the work done by the worker is given, so we have \begin{align*} W&=Fd\cos\theta\\&=(45)(50) \cos 32^\circ\\&=1912.5\quad {\rm J}\end{align*} (b) ''net'' means "total". In all work problems in physics, there are two equivalent methods to find the net work. Identify all forces that are applied to the cart, find their resultant force, and then compute the work done by this net force over a specific distance.

Or compute all works done on the object across a distance individually, then sum them algebraically.

Usually, the second method is easier. We take this approach here.

The cart moves in a straight horizontal path. All forces apply on it are, the worker force $F$, the normal force $F_N$, and the force of gravity or its weight $F_g=mg$. The work done by normal and gravity forces in a horizontal displacement is always zero since the angle between these forces and the displacement is $90^\circ$. So, $W_N=W_g=0$. Hence, the net (total) work done on the object is \[W_{total}=W_N+W_g+W_F=1912.5\,{\rm J}\]

Problem (6): A $1200-{\rm kg}$ box is at rest on a rough floor. How much work is required to move it $5\,{\rm m}$ at a constant speed (a) along the floor against a $230\,{\rm N}$ friction force, (b) vertically?

Solution : In this problem, we want to displace a box $5\,{\rm m}$ horizontally and vertically. In the horizontal direction, there is also kinetic friction.

(a) At constant speed , means there is no acceleration in the course of displacement, so according to Newton's second law $\Sigma F=ma$, the net force on the box must be zero. To meet this condition, the external force $F_p$ applied by a person must cancel out the friction force $f_k$. So, \[F_p=f_k=230\quad {\rm N}\] The force $F_p$ and displacement are both parallel, so their product get the work done by $F_p$ \[W_p=F_p d=230\times 5=1150\quad {\rm J}\] (b) In the vertical path, two forces act on the box. One is the external lifting force, and the other is the force of gravity. Since the box is moving at constant speed vertically, there is no acceleration, and thus this lifting external force $F_p$ must be balanced with the weight of the box. \[F_p=F_g=mg=(1200)(10)=12000\,{\rm J}\] Assume the box is moved vertically upward. In this case, the lifting force and displacement are parallel, so the angle between them is zero $\theta=0$, and the work done by this force is \[W_p=F_p d\cos 0=12000\times 5=60\,{\rm kJ}\] On the other side, the weight force, or force of gravity $F_g=mg$ is always downward, so the angle between the box's weight and upward displacement is $180^\circ$. So, the work done by the weight of the box is \begin{align*}W_g&=F_g d\cos 180^\circ \\\\ &=(1200)(10)(5)(-1) \\\\ &=-60\,{\rm kJ}\end{align*} In such cases where the angle between $F$ and $d$ is $180^\circ$, they are called antiparallel.

Problem (7): A 40-kg crate is pushed using a force of 150 N at a distance of $6\,{\rm m}$ on a rough surface. The crate moves at a constant speed. Find (a) the work done by the external force on the crate. (b) The coefficient of kinetic friction between the crate and the floor?

Solution : (a) the crate is moved horizontally through a distance of $6\,{\rm m}$ by a force parallel to its displacement. So, the work done by this external force is \[W_p=F_p d \cos\theta=(150)(6)\cos 0=900\,{\rm J}\] where subscript $p$ denotes the person or any external agent.

(b) According to the definition of the kinetic friction force formula, $f_k=\mu_k F_N$, to find the coefficient of kinetic friction $\mu_k$, we must have both the friction force and normal force $F_N$.

In the question, we are told that the crate moves at a constant speed, so there is no acceleration, and thus, the net force applied to it must be zero.

When the friction force, which opposes the motion, is equal to the external force $F_p$, then this condition is satisfied. So, \[f_k=F_p=150\,{\rm N}\] On the other side, the crate is not lifted off the floor, so there is no motion vertically.

Balancing all forces applied vertically, the weight force and the normal force $F_N$, we can find the normal force $F_N$ as below \begin{gather*} F_N-F_g=0\\ F_N=F_g\\ \Rightarrow F_N=mg=40\times 10=400\quad {\rm N}\end{gather*} Therefore, the coefficient of kinetic friction is found to be \[f_k=\frac{f_k}{F_N}=\frac{150}{400}=0.375\]

Practice these questions to understand friction force Problems on the coefficient of friction

Problem (8): A 18-kg packing box is pulled at constant speed by a rope inclined at $20^\circ$. The box moves a distance of 20 m over a rough horizontal surface. Assume the coefficient of kinetic friction between the box and the surface to be $0.5$. (a) Find the tension in the rope? (b) How much work is done by the rope on the box?

Solution : The aim of this problem is to find the work done by the tension in the rope. The magnitude of the tension in the rope is not given. So, we must first find it.

(a) We are told the box moves at a constant speed, so, as previously mentioned, the net force on the box must be zero to produce no acceleration. But what forces are acting horizontally on the box? The horizontal component of tension in the rope, $T_{\parallel}=T\cos\theta$, and the kinetic friction force $f_k$ in the opposite direction of motion are the forces acting on the box horizontally.

If these two forces are equal in magnitude but opposite in direction, then their resultant (net) becomes zero, and consequently, the box will move at a constant speed. \begin{align*} f_k&=T_{\parallel}\\\mu_k F_N&=T\cos\theta\quad (I) \end{align*} The forces in the vertical direction must also cancel each other since there is no motion vertically. As you can see in the figure, we have \[F_N=T\sin\theta+F_g\] Substituting this into the relation (I), rearranging and solving for $T$, yields \begin{gather*} \mu_k (T\sin\theta+mg)=T\cos\theta \\\\ \Rightarrow T=\frac{\mu_k mg}{\cos\theta-\mu_k\sin\theta}\end{gather*} Substituting the numerical values into the above expression, we find the tension in the rope. \[T=\frac{(0.5)(18)(10)}{\cos 20^\circ-(0.5) \sin20^\circ}=117\quad {\rm N}\] (b) The only force that causes the box to move some distance is the horizontal component of the tension in the rope, $T_{\parallel}=T\cos\theta$. So, the work done by the tension in the rope is \begin{align*} W&=T_{\parallel}d\\ &=(117)( \cos 20^\circ)(20) \\&=2199\quad {\rm J}\end{align*}

Problem (9): A table of mass 40 kg is accelerated from rest at a constant rate of $2\,{\rm m/s^2}$ for $4\,{\rm s}$ by a constant force. What is the net work done on the table?

Solution : This is a combination of a kinematics problem and a physics work problem. Here, first, we must find the distance over which the box is displaced. The given information is: initial speed $v_0=0$, acceleration $2\,{\rm m/s^2}$, time taken $t=4\,{\rm s}$. Using this data, we can find the total displacement by applying the kinematics equation $\Delta x=\frac 12 at^2+v_0t$, \begin{align*} \Delta x&=\frac 12 at^2+v_0t\\\\&=\frac 12 (2)(4)^2+0(4)\\\\&=16\quad {\rm m}\end{align*} So, this constant force causes the table to move a distance of 16 meters across the surface. To find the work done, we need a force, as well. The force is mass times acceleration, $F=ma$, so we have \[F=ma=40\times 2=80\,{\rm N}\] Now that we have both the force and displacement, the net work done on the table is the product of force along the displacement times the magnitude of displacement \[W=80\times 16=128\quad {\rm J}\]

Work problems in a uniform circular motion

Problem (10): A 5-kg object is held at the end of a string and undergoes uniform circular motion around a circle of radius $5\,{\rm m}$. If the tangential speed of the object around the circle is $15\,{\rm m/s}$, how much work was done on the object by the centripetal force?

Solution : Here, an object moves around a circle, so we encounter a uniform circular motion problem .

In such motions around a curve or circle, that force in the radial direction exerting on the object is called the centripetal force.

On the other hand, a movement around a circle is tangent to the path at any instant of time. Thus, we conclude that in any uniform circular motion, a force is applied to the whirling object that is perpendicular to its motion at any moment of time.

So, the angle between the centripetal force and displacement at any instant is always zero, $\theta=0$. Using the work formula $W=Fd\cos\theta$, we find that the work done by the centripetal force is always zero.

This is another example of zero work in physics.

Work problems on an incline

Problem (11): A $5-{\rm kg}$ box, initially at rest, slides $2.5\,{\rm m}$ down a ramp of angle $30^\circ$. The coefficient of friction between the box and the incline is $\mu_k=0.435$. Determine (a) the work done by the gravity force, (b) the work done by the frictional force, and (c) the work done by the normal force exerted by the surface.

Solution : This part is related to problems on inclined plane surfaces . The forces acting on a box on an inclined plane are shown in the figure. As you can see, the forces along the direction of motion are the parallel component of the weight $W_{\parallel}$, and the friction force $f_k$.

(a) In the figure, you realize that the angle between the object's weight (the same as the force of gravity) and downward displacement $d$ is zero, $\theta=30^\circ$.

So, the work done by the force of gravity on the box is found using work formula as below \begin{align*} W&=Fd\cos\theta\\&=(mg)(d) \cos 30^\circ\\&=(5\times 10)(2.5) \cos 30^\circ\\&=108.25\quad {\rm J}\end{align*} (b) To find the work done by friction, we need to know its magnitude. From the kinetic friction force formula, $f_k=\mu_k F_N$, we must determine, first, the normal force acting on the box.

There is no motion in the direction perpendicular to the incline, so the resultant of forces acting in this direction must be zero. Equating the same direction forces, we will have \[F_N=mg\sin\alpha=(5)(10) \sin 30^\circ=25\,{\rm N}\] Substituting this into the above equation for kinetic friction, we can find its magnitude as \[f_k=\mu_k F_N=(0.435)(25)=10.875\,{\rm N}\] The friction force and the displacement of the box down the ramp are parallel, i.e., $\theta=0$. The work done by friction is \begin{align*} W_f&=f_kd\cos\theta \\\\ &=(10.875)(2.5) \cos 0\\\\ &=27.1875\,{\rm J}\end{align*} (c) By definition, the normal force is the same contact force that is applied to the object from the surface perpendicularly. On the other hand, the object moves along the incline, so its displacement is perpendicular to the normal force, $\theta=90^\circ$. Hence, the work done by the normal force is zero. \[W_N=F_N d\cos\theta=F_N d\cos 90^\circ=0\]

Problem (12): We want to push a $950-{\rm kg}$ heavy object 650 m up along a $7^\circ$ incline at a constant speed. How much work do we do over this distance? Ignore friction.

Solution : When it comes to constant speed in all work problems in physics, you must remember that all forces in the same direction must be equal to the opposing forces. This condition ensures that there is no acceleration in the motion.

In this case, all forces acting on the object are: the pushing force along the incline upward $F_p$, and the parallel component of the force of gravity (weight) along the incline downward, $W_{\parallel}=mg\sin\alpha$. Thus, we can find the pushing force as \begin{align*}F_p&=mg\sin 7^\circ\\\\ &=(950)(10) \sin 7^\circ \\\\& =1159\quad {\rm N}\end{align*} In the question, we are told that the object is moving up the incline, so the angle between its displacement and upward pushing force is zero, $\theta=0$. Hence, the work done by the person to push the object along the incline upward is \begin{align*} W_p&=F_p d\cos\theta\\&=1159\times 650 \cos 0\\&=753350\quad{\rm J}\end{align*}

Problem (13): Consider an electron moving at a constant speed of $1.1\times 10^6\,\rm m/s$ in a straight line. How much energy is required to stop this electron? (Take the electron's mass, $m_e=9.11\times 10^{-31}\,\rm kg$.

Solution : In this problem on work, we cannot use the work formula directly, since none of the work variables, i.e., $F$, $d$, $\theta$, are given except the velocity. In these cases, we have a problem on the work-energy theorem .

According to this rule, the net work done over a distance by a constant force on an object of mass $m$ equals the change in its kinetic energy \[W_{net}=\underbrace{\frac 12 mv_f^2-\frac 12 mv_i^2}_{\Delta K}\] Substituting the numerical values given in this problem, we get the required work to stop this fast-moving electron. \begin{align*}W_{net}&=\frac 12 m(v_f^2-v_i^2) \\\\ &=\frac 12 (9.11\times 10^{-31}) \left(0^2-(1.1\times 10^6)^2 \right) \\\\ &=-5.5\times 10^{-19}\,\rm J\end{align*} where we set the final velocity $v_f=0$ since the electron is to stop.

Problem (14): How much power is needed to lift a $25-\rm kg$ weight $1\,\rm m$ in $1\,\rm s$?

Solution : The power in physics is defined as the ratio of work done on an object to the time taken $P=\frac{W}{t}$. The SI unit of power is the watt ($W$).

In this problem, first, we must find the amount of work done in lifting the object as much as $1,\rm m$ vertically. The only force involved in this situation is the downward weight force. Thus, \[W=(mg)h=(25\times 10)(1)=250\,{\rm J}\] This amount of work has been done in a time interval of $1\,\rm s$. Hence, the power is calculated as below \[P=\frac{W}{t}=\frac{250}{1}=250\,\rm W\]

Problem (15): A particle having charge $-3.6\,\rm nC$ is released from rest in a uniform electric field $E$ moves a distance of $5\,\rm cm$ through it. The electric potential difference between those two points is $\Delta V=+400\,\rm V$. What work was done by the electric force on the particle?

Solution : The work done by the electric force on a charged particle is calculated by $W_E=qEd$, where $E$ is the magnitude of the electric field and $d$ is the amount of distance traveled through $E$. But in this case, the electric field strength is not given, and we cannot use this formula.

We can see this as a problem on electric potential . Recall that the work done by the electric force on a charge to move it between two points with different potentials is given by $W=-q\Delta V$. Substituting the given numerical values into this, we will have \begin{align*} W&=-q\Delta V \\&=-(-3.6)(+400) \\&=\boxed{1440\,\rm J} \end{align*}

Here, we learned how to calculate the work done by a constant force in physics by solving a couple of example problems.

Overall, the work done by a constant force is the product of the horizontal component of the force times the displacement between the initial and final points.

In addition, power, a related quantity to work in physics, is also defined as the rate at which work is done.

Author : Dr. Ali Nemati Date Published : 9/20/2021

Problems & Exercises
Introduction to Science and the Realm of Physics, Physical Quantities, and Units
1.1 Physics: An Introduction

1.2 Physical Quantities and Units

1.3 accuracy, precision, and significant figures, 1.4 approximation.

Section Summary
Conceptual Questions
Introduction to One-Dimensional Kinematics
2.1 Displacement
2.2 Vectors, Scalars, and Coordinate Systems
2.3 Time, Velocity, and Speed
2.4 Acceleration
2.5 Motion Equations for Constant Acceleration in One Dimension
2.6 Problem-Solving Basics for One-Dimensional Kinematics
2.7 Falling Objects
2.8 Graphical Analysis of One-Dimensional Motion
Introduction to Two-Dimensional Kinematics
3.1 Kinematics in Two Dimensions: An Introduction
3.2 Vector Addition and Subtraction: Graphical Methods
3.3 Vector Addition and Subtraction: Analytical Methods
3.4 Projectile Motion
3.5 Addition of Velocities
Introduction to Dynamics: Newton’s Laws of Motion
4.1 Development of Force Concept
4.2 Newton’s First Law of Motion: Inertia
4.3 Newton’s Second Law of Motion: Concept of a System
4.4 Newton’s Third Law of Motion: Symmetry in Forces
4.5 Normal, Tension, and Other Examples of Forces
4.6 Problem-Solving Strategies
4.7 Further Applications of Newton’s Laws of Motion
4.8 Extended Topic: The Four Basic Forces—An Introduction
Introduction: Further Applications of Newton’s Laws
5.1 Friction
5.2 Drag Forces
5.3 Elasticity: Stress and Strain
Introduction to Uniform Circular Motion and Gravitation
6.1 Rotation Angle and Angular Velocity
6.2 Centripetal Acceleration
6.3 Centripetal Force
6.4 Fictitious Forces and Non-inertial Frames: The Coriolis Force
6.5 Newton’s Universal Law of Gravitation
6.6 Satellites and Kepler’s Laws: An Argument for Simplicity
Introduction to Work, Energy, and Energy Resources
7.1 Work: The Scientific Definition
7.2 Kinetic Energy and the Work-Energy Theorem
7.3 Gravitational Potential Energy
7.4 Conservative Forces and Potential Energy
7.5 Nonconservative Forces
7.6 Conservation of Energy
7.8 Work, Energy, and Power in Humans
7.9 World Energy Use
Introduction to Linear Momentum and Collisions
8.1 Linear Momentum and Force
8.2 Impulse
8.3 Conservation of Momentum
8.4 Elastic Collisions in One Dimension
8.5 Inelastic Collisions in One Dimension
8.6 Collisions of Point Masses in Two Dimensions
8.7 Introduction to Rocket Propulsion
Introduction to Statics and Torque
9.1 The First Condition for Equilibrium
9.2 The Second Condition for Equilibrium
9.3 Stability
9.4 Applications of Statics, Including Problem-Solving Strategies
9.5 Simple Machines
9.6 Forces and Torques in Muscles and Joints
Introduction to Rotational Motion and Angular Momentum
10.1 Angular Acceleration
10.2 Kinematics of Rotational Motion
10.3 Dynamics of Rotational Motion: Rotational Inertia
10.4 Rotational Kinetic Energy: Work and Energy Revisited
10.5 Angular Momentum and Its Conservation
10.6 Collisions of Extended Bodies in Two Dimensions
10.7 Gyroscopic Effects: Vector Aspects of Angular Momentum
Introduction to Fluid Statics
11.1 What Is a Fluid?
11.2 Density
11.3 Pressure
11.4 Variation of Pressure with Depth in a Fluid
11.5 Pascal’s Principle
11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement
11.7 Archimedes’ Principle
11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action
11.9 Pressures in the Body
Introduction to Fluid Dynamics and Its Biological and Medical Applications
12.1 Flow Rate and Its Relation to Velocity
12.2 Bernoulli’s Equation
12.3 The Most General Applications of Bernoulli’s Equation
12.4 Viscosity and Laminar Flow; Poiseuille’s Law
12.5 The Onset of Turbulence
12.6 Motion of an Object in a Viscous Fluid
12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes
Introduction to Temperature, Kinetic Theory, and the Gas Laws
13.1 Temperature
13.2 Thermal Expansion of Solids and Liquids
13.3 The Ideal Gas Law
13.4 Kinetic Theory: Atomic and Molecular Explanation of Pressure and Temperature
13.5 Phase Changes
13.6 Humidity, Evaporation, and Boiling
Introduction to Heat and Heat Transfer Methods
14.2 Temperature Change and Heat Capacity
14.3 Phase Change and Latent Heat
14.4 Heat Transfer Methods
14.5 Conduction
14.6 Convection
14.7 Radiation
Introduction to Thermodynamics
15.1 The First Law of Thermodynamics
15.2 The First Law of Thermodynamics and Some Simple Processes
15.3 Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency
15.4 Carnot’s Perfect Heat Engine: The Second Law of Thermodynamics Restated
15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators
15.6 Entropy and the Second Law of Thermodynamics: Disorder and the Unavailability of Energy
15.7 Statistical Interpretation of Entropy and the Second Law of Thermodynamics: The Underlying Explanation
Introduction to Oscillatory Motion and Waves
16.1 Hooke’s Law: Stress and Strain Revisited
16.2 Period and Frequency in Oscillations
16.3 Simple Harmonic Motion: A Special Periodic Motion
16.4 The Simple Pendulum
16.5 Energy and the Simple Harmonic Oscillator
16.6 Uniform Circular Motion and Simple Harmonic Motion
16.7 Damped Harmonic Motion
16.8 Forced Oscillations and Resonance
16.10 Superposition and Interference
16.11 Energy in Waves: Intensity
Introduction to the Physics of Hearing
17.2 Speed of Sound, Frequency, and Wavelength
17.3 Sound Intensity and Sound Level
17.4 Doppler Effect and Sonic Booms
17.5 Sound Interference and Resonance: Standing Waves in Air Columns
17.6 Hearing
17.7 Ultrasound
Introduction to Electric Charge and Electric Field
18.1 Static Electricity and Charge: Conservation of Charge
18.2 Conductors and Insulators
18.3 Coulomb’s Law
18.4 Electric Field: Concept of a Field Revisited
18.5 Electric Field Lines: Multiple Charges
18.6 Electric Forces in Biology
18.7 Conductors and Electric Fields in Static Equilibrium
18.8 Applications of Electrostatics
Introduction to Electric Potential and Electric Energy
19.1 Electric Potential Energy: Potential Difference
19.2 Electric Potential in a Uniform Electric Field
19.3 Electrical Potential Due to a Point Charge
19.4 Equipotential Lines
19.5 Capacitors and Dielectrics
19.6 Capacitors in Series and Parallel
19.7 Energy Stored in Capacitors
Introduction to Electric Current, Resistance, and Ohm's Law
20.1 Current
20.2 Ohm’s Law: Resistance and Simple Circuits
20.3 Resistance and Resistivity
20.4 Electric Power and Energy
20.5 Alternating Current versus Direct Current
20.6 Electric Hazards and the Human Body
20.7 Nerve Conduction–Electrocardiograms
Introduction to Circuits and DC Instruments
21.1 Resistors in Series and Parallel
21.2 Electromotive Force: Terminal Voltage
21.3 Kirchhoff’s Rules
21.4 DC Voltmeters and Ammeters
21.5 Null Measurements
21.6 DC Circuits Containing Resistors and Capacitors
Introduction to Magnetism
22.1 Magnets
22.2 Ferromagnets and Electromagnets
22.3 Magnetic Fields and Magnetic Field Lines
22.4 Magnetic Field Strength: Force on a Moving Charge in a Magnetic Field
22.5 Force on a Moving Charge in a Magnetic Field: Examples and Applications
22.6 The Hall Effect
22.7 Magnetic Force on a Current-Carrying Conductor
22.8 Torque on a Current Loop: Motors and Meters
22.9 Magnetic Fields Produced by Currents: Ampere’s Law
22.10 Magnetic Force between Two Parallel Conductors
22.11 More Applications of Magnetism
Introduction to Electromagnetic Induction, AC Circuits and Electrical Technologies
23.1 Induced Emf and Magnetic Flux
23.2 Faraday’s Law of Induction: Lenz’s Law
23.3 Motional Emf
23.4 Eddy Currents and Magnetic Damping
23.5 Electric Generators
23.6 Back Emf
23.7 Transformers
23.8 Electrical Safety: Systems and Devices
23.9 Inductance
23.10 RL Circuits
23.11 Reactance, Inductive and Capacitive
23.12 RLC Series AC Circuits
Introduction to Electromagnetic Waves
24.1 Maxwell’s Equations: Electromagnetic Waves Predicted and Observed
24.2 Production of Electromagnetic Waves
24.3 The Electromagnetic Spectrum
24.4 Energy in Electromagnetic Waves
Introduction to Geometric Optics
25.1 The Ray Aspect of Light
25.2 The Law of Reflection
25.3 The Law of Refraction
25.4 Total Internal Reflection
25.5 Dispersion: The Rainbow and Prisms
25.6 Image Formation by Lenses
25.7 Image Formation by Mirrors
Introduction to Vision and Optical Instruments
26.1 Physics of the Eye
26.2 Vision Correction
26.3 Color and Color Vision
26.4 Microscopes
26.5 Telescopes
26.6 Aberrations
Introduction to Wave Optics
27.1 The Wave Aspect of Light: Interference
27.2 Huygens's Principle: Diffraction
27.3 Young’s Double Slit Experiment
27.4 Multiple Slit Diffraction
27.5 Single Slit Diffraction
27.6 Limits of Resolution: The Rayleigh Criterion
27.7 Thin Film Interference
27.8 Polarization
27.9 *Extended Topic* Microscopy Enhanced by the Wave Characteristics of Light
Introduction to Special Relativity
28.1 Einstein’s Postulates
28.2 Simultaneity And Time Dilation
28.3 Length Contraction
28.4 Relativistic Addition of Velocities
28.5 Relativistic Momentum
28.6 Relativistic Energy
Introduction to Quantum Physics
29.1 Quantization of Energy
29.2 The Photoelectric Effect
29.3 Photon Energies and the Electromagnetic Spectrum
29.4 Photon Momentum
29.5 The Particle-Wave Duality
29.6 The Wave Nature of Matter
29.7 Probability: The Heisenberg Uncertainty Principle
29.8 The Particle-Wave Duality Reviewed
Introduction to Atomic Physics
30.1 Discovery of the Atom
30.2 Discovery of the Parts of the Atom: Electrons and Nuclei
30.3 Bohr’s Theory of the Hydrogen Atom
30.4 X Rays: Atomic Origins and Applications
30.5 Applications of Atomic Excitations and De-Excitations
30.6 The Wave Nature of Matter Causes Quantization
30.7 Patterns in Spectra Reveal More Quantization
30.8 Quantum Numbers and Rules
30.9 The Pauli Exclusion Principle
Introduction to Radioactivity and Nuclear Physics
31.1 Nuclear Radioactivity
31.2 Radiation Detection and Detectors
31.3 Substructure of the Nucleus
31.4 Nuclear Decay and Conservation Laws
31.5 Half-Life and Activity
31.6 Binding Energy
31.7 Tunneling
Introduction to Applications of Nuclear Physics
32.1 Medical Imaging and Diagnostics
32.2 Biological Effects of Ionizing Radiation
32.3 Therapeutic Uses of Ionizing Radiation
32.4 Food Irradiation
32.5 Fusion
32.6 Fission
32.7 Nuclear Weapons
Introduction to Particle Physics
33.1 The Yukawa Particle and the Heisenberg Uncertainty Principle Revisited
33.2 The Four Basic Forces
33.3 Accelerators Create Matter from Energy
33.4 Particles, Patterns, and Conservation Laws
33.5 Quarks: Is That All There Is?
33.6 GUTs: The Unification of Forces
Introduction to Frontiers of Physics
34.1 Cosmology and Particle Physics
34.2 General Relativity and Quantum Gravity
34.3 Superstrings
34.4 Dark Matter and Closure
34.5 Complexity and Chaos
34.6 High-temperature Superconductors
34.7 Some Questions We Know to Ask
A | Atomic Masses
B | Selected Radioactive Isotopes
C | Useful Information
D | Glossary of Key Symbols and Notation

The speed limit on some interstate highways is roughly 100 km/h. (a) What is this in meters per second? (b) How many miles per hour is this?

A car is traveling at a speed of 33 m/s 33 m/s size 12{"33"" m/s"} {} . (a) What is its speed in kilometers per hour? (b) Is it exceeding the 90 km/h 90 km/h size 12{"90"" km/h"} {} speed limit?

Show that 1 . 0 m/s = 3 . 6 km/h 1 . 0 m/s = 3 . 6 km/h size 12{1 "." 0`"m/s"=3 "." "6 km/h"} {} . Hint: Show the explicit steps involved in converting 1 . 0 m/s = 3 . 6 km/h. 1 . 0 m/s = 3 . 6 km/h. size 12{1 "." 0`"m/s"=3 "." "6 km/h"} {}

American football is played on a 100-yd-long field, excluding the end zones. How long is the field in meters? (Assume that 1 meter equals 3.281 feet.)

Soccer fields vary in size. A large soccer field is 115 m long and 85 m wide. What are its dimensions in feet and inches? (Assume that 1 meter equals 3.281 feet.)

What is the height in meters of a person who is 6 ft 1.0 in. tall? (Assume that 1 meter equals 39.37 in.)

Mount Everest, at 29,028 feet, is the tallest mountain on the Earth. What is its height in kilometers? (Assume that 1 kilometer equals 3,281 feet.)

The speed of sound is measured to be 342 m/s 342 m/s size 12{"342"" m/s"} {} on a certain day. What is this in km/h?

Tectonic plates are large segments of the Earth’s crust that move slowly. Suppose that one such plate has an average speed of 4.0 cm/year. (a) What distance does it move in 1 s at this speed? (b) What is its speed in kilometers per million years?

(a) Refer to Table 1.3 to determine the average distance between the Earth and the Sun. Then calculate the average speed of the Earth in its orbit in kilometers per second. (b) What is this in meters per second?

Express your answers to problems in this section to the correct number of significant figures and proper units.

Suppose that your bathroom scale reads your mass as 65 kg with a 3% uncertainty. What is the uncertainty in your mass (in kilograms)?

A good-quality measuring tape can be off by 0.50 cm over a distance of 20 m. What is its percent uncertainty?

(a) A car speedometer has a 5.0 % 5.0 % size 12{5.0%} {} uncertainty. What is the range of possible speeds when it reads 90 km/h 90 km/h size 12{"90"" km/h"} {} ? (b) Convert this range to miles per hour. 1 km = 0.6214 mi 1 km = 0.6214 mi size 12{"1 km" "=" "0.6214 mi"} {}

An infant’s pulse rate is measured to be 130 ± 5 130 ± 5 size 12{"130" +- 5} {} beats/min. What is the percent uncertainty in this measurement?

(a) Suppose that a person has an average heart rate of 72.0 beats/min. How many beats does he or she have in 2.0 y? (b) In 2.00 y? (c) In 2.000 y?

A can contains 375 mL of soda. How much is left after 308 mL is removed?

State how many significant figures are proper in the results of the following calculations: (a) 106 . 7 98 . 2 / 46 . 210 1 . 01 106 . 7 98 . 2 / 46 . 210 1 . 01 size 12{ left ("106" "." 7 right ) left ("98" "." 2 right )/ left ("46" "." "210" right ) left (1 "." "01" right )} {} (b) 18 . 7 2 18 . 7 2 size 12{ left ("18" "." 7 right ) rSup { size 8{2} } } {} (c) 1 . 60 × 10 − 19 3712 1 . 60 × 10 − 19 3712 size 12{ left (1 "." "60" times "10" rSup { size 8{ - "19"} } right ) left ("3712" right )} {} .

(a) How many significant figures are in the numbers 99 and 100? (b) If the uncertainty in each number is 1, what is the percent uncertainty in each? (c) Which is a more meaningful way to express the accuracy of these two numbers, significant figures or percent uncertainties?

(a) If your speedometer has an uncertainty of 2 . 0 km/h 2 . 0 km/h size 12{2 "." 0" km/h"} {} at a speed of 90 km/h 90 km/h size 12{"90"" km/h"} {} , what is the percent uncertainty? (b) If it has the same percent uncertainty when it reads 60 km/h 60 km/h size 12{"60"" km/h"} {} , what is the range of speeds you could be going?

(a) A person’s blood pressure is measured to be 120 ± 2 mm Hg 120 ± 2 mm Hg size 12{"120" +- 2" mm Hg"} {} . What is its percent uncertainty? (b) Assuming the same percent uncertainty, what is the uncertainty in a blood pressure measurement of 80 mm Hg 80 mm Hg size 12{"80"" mm Hg"} {} ?

A person measures his or her heart rate by counting the number of beats in 30 s 30 s size 12{"30"" s"} {} . If 40 ± 1 40 ± 1 size 12{"40" +- 1} {} beats are counted in 30 . 0 ± 0 . 5 s 30 . 0 ± 0 . 5 s size 12{"30" "." 0 +- 0 "." 5" s"} {} , what is the heart rate and its uncertainty in beats per minute?

What is the area of a circle 3 . 102 cm 3 . 102 cm size 12{3 "." "102"" cm"} {} in diameter?

If a marathon runner averages 9.5 mi/h, how long does it take him or her to run a 26.22-mi marathon?

A marathon runner completes a 42 . 188 -km 42 . 188 -km size 12{"42" "." "188""-km"} {} course in 2 h 2 h size 12{2" h"} {} , 30 min, and 12 s 12 s size 12{"12"" s"} {} . There is an uncertainty of 25 m 25 m size 12{"25"" m"} {} in the distance traveled and an uncertainty of 1 s in the elapsed time. (a) Calculate the percent uncertainty in the distance. (b) Calculate the uncertainty in the elapsed time. (c) What is the average speed in meters per second? (d) What is the uncertainty in the average speed?

The sides of a small rectangular box are measured to be 1 . 80 ± 0 . 01 cm 1 . 80 ± 0 . 01 cm size 12{1 "." "80" +- 0 "." "01"" cm"} {} , {} 2 . 05 ± 0 . 02 cm, and 3 . 1 ± 0 . 1 cm 2 . 05 ± 0 . 02 cm, and 3 . 1 ± 0 . 1 cm size 12{2 "." "05" +- 0 "." "02"" cm, and 3" "." 1 +- 0 "." "1 cm"} {} long. Calculate its volume and uncertainty in cubic centimeters.

When non-metric units were used in the United Kingdom, a unit of mass called the pound-mass (lbm) was employed, where 1 lbm = 0 . 4539 kg 1 lbm = 0 . 4539 kg size 12{1" lbm"=0 "." "4539"`"kg"} {} . (a) If there is an uncertainty of 0 . 0001 kg 0 . 0001 kg size 12{0 "." "0001"`"kg"} {} in the pound-mass unit, what is its percent uncertainty? (b) Based on that percent uncertainty, what mass in pound-mass has an uncertainty of 1 kg when converted to kilograms?

The length and width of a rectangular room are measured to be 3 . 955 ± 0 . 005 m 3 . 955 ± 0 . 005 m size 12{3 "." "955" +- 0 "." "005"" m"} {} and 3 . 050 ± 0 . 005 m 3 . 050 ± 0 . 005 m size 12{3 "." "050" +- 0 "." "005"" m"} {} . Calculate the area of the room and its uncertainty in square meters.

A car engine moves a piston with a circular cross section of 7 . 500 ± 0 . 002 cm 7 . 500 ± 0 . 002 cm size 12{7 "." "500" +- 0 "." "002"`"cm"} {} diameter a distance of 3 . 250 ± 0 . 001 cm 3 . 250 ± 0 . 001 cm size 12{3 "." "250" +- 0 "." "001"`"cm"} {} to compress the gas in the cylinder. (a) By what amount is the gas decreased in volume in cubic centimeters? (b) Find the uncertainty in this volume.

How many heartbeats are there in a lifetime?

A generation is about one-third of a lifetime. Approximately how many generations have passed since the year 0 AD?

How many times longer than the mean life of an extremely unstable atomic nucleus is the lifetime of a human? (Hint: The lifetime of an unstable atomic nucleus is on the order of 10 − 22 s 10 − 22 s size 12{"10" rSup { size 8{ - "22"} } " s"} {} .)

Calculate the approximate number of atoms in a bacterium. Assume that the average mass of an atom in the bacterium is ten times the mass of a hydrogen atom. (Hint: The mass of a hydrogen atom is on the order of 10 − 27 kg 10 − 27 kg size 12{"10" rSup { size 8{ - "27"} } " kg"} {} and the mass of a bacterium is on the order of 10 − 15 kg. 10 − 15 kg. size 12{"10" rSup { size 8{ - "15"} } "kg"} {} )

A magnified image of the bacterium Salmonella attacking a human cell. The bacterium is rod shaped and about zero point seven to one point five micrometers in diameter and two to five micrometers in length.

Approximately how many atoms thick is a cell membrane, assuming all atoms there average about twice the size of a hydrogen atom?

(a) What fraction of Earth’s diameter is the greatest ocean depth? (b) The greatest mountain height?

(a) Calculate the number of cells in a hummingbird assuming the mass of an average cell is ten times the mass of a bacterium. (b) Making the same assumption, how many cells are there in a human?

Assuming one nerve impulse must end before another can begin, what is the maximum firing rate of a nerve in impulses per second?

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Access for free at https://openstax.org/books/college-physics/pages/1-introduction-to-science-and-the-realm-of-physics-physical-quantities-and-units

Authors: Paul Peter Urone, Roger Hinrichs
Publisher/website: OpenStax
Book title: College Physics
Publication date: Jun 21, 2012
Location: Houston, Texas
Book URL: https://openstax.org/books/college-physics/pages/1-introduction-to-science-and-the-realm-of-physics-physical-quantities-and-units
Section URL: https://openstax.org/books/college-physics/pages/1-problems-exercises

© Mar 3, 2022 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.

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High School Physics : Calculating Work

Study concepts, example questions & explanations for high school physics, all high school physics resources, example questions, example question #1 : calculating work.