[CQ] Physics Honors - Work and Energy

Yes/No: Is pushing a shopping cart forward an example of when an object moves in the same direction as the direction of the force, causing work to be positive?

Yes

The energy of a system will change if...
A. work is done on the system by an internal force.
B. work is done on the system by an external force.
C. no work is done on the system.

B

When the work done on a system is positive...
A. the energy of the system will increase.
B. the energy of the system will decrease.
C. the energy of the system will remain constant (conserved).

A

When the word done on a system is negative...
A. the energy of the system will increase.
B. the energy of the system will decrease.
C. the energy of the system will remain constant (conserved).

B

Energy is a...
A. Vector (Magnitude + Direction).
B. Scalar (Only Magnitude).

B

Leena loads a Nerf ball into a spring-loaded Nerf gun. Assume all frictional forces are negligible. What type of energy does the ball have at this point?

EPE

Leena shoots a Nerf ball straight up into the air. Assume all frictional forces are negligible. What type of energy does the ball have as it is leaving the gun?

KE

A ball reaches a maximum height of 2 meters. Assume all frictional forces are negligible. At this point, what type of energy does the ball have?

GPE

If a ball started with 0.6 J of energy while it was loaded in a Nerf gun, how much kinetic energy did it have as it left the gun? Assume all frictional forces are negligible.

0.6 J

At the highest point of 2 m, how much GPE did a ball being shot from a Nerf gun have? Assume all frictional forces are negligible.

0.6 J

System

The piece of the universe that we look at when we solve a specific problem.

Environment

Everything that is not a part of the system.

Open Systems

A system that can be affected by forces from the environment. These forces are also known as external forces.

Closed Systems

Systems that cannot be affected by forces from the environment.

What kind of system is Earth?

Open

How do external forces affect an open system?

External forces do work on an open system, which in turn changes the energy of the system.

If a system has some initial energy, Eᵢ, doing work on it will change the energy (either adding or taking away energy), to make a new value, Ef. What formula does this result in?

Eᵢ + W = Ef

True/False: External forces can affect a closed system.

False

True/False: External forces can do work on a closed system.

False

Energy will remain unchanged or conserved in a closed system. What equation does this result in?

Eᵢ = Ef

The amount of work done and therefore the amount of energy increase that the system will experience is given by what equation?

W = Fdₚₐᵣₐₗₗₑₗ

Yes/No: If you apply a force to a wall and the wall doesn't move, did you do work?

No

What happens to the energy of a system if an object moves in the same direction as the direction of the force, causing work to be positive?

It increases.

What happens to the energy of a system if an object moves in the direction opposite of the direction of the force, causing the work to be negative?

It decreases.

What happens to the energy of a system if an object moves in the direction perpendicular to the direction of the force, causing the work to be zero?

It is unchanged.

What is the unit for work?

Joules (J)

How do you find the work done on a graph where the y-axis represents the force and the x-axis represents the displacement?

Find the area under the line.

Ginny lifts a barbell of mass "m" vertically upwards a distance "h" at a constant velocity. How much work does Ginny do on the barbell?

mgh J

Gravitational Potential Energy (GPE)

The energy of an object as a result of its position relative to the Earth's surface. The formula used to find this is mgh.

As an object falls downward, its GPE always __________________.
A. increases
B. decreases
C. stays the same

B

Leonard holds a ball at a height of 2 meters and then lets go. What type of energy does the ball have initially?

GPE

Leonard holds a ball at a height of 2 meters and then lets go. What type of energy does it have right before it hits the floor?

KE

Kinetic Energy (KE)

The energy an object has by virtue of its motion. The formula used to find this is ½mv².

As an object falls, its KE always __________________.
A. decreases
B. increases
C. stays the same
D. changes direction

B

A ball falls from the top of a building to the ground below. How does the kinetic energy compare to the gravitational potential energy at the top of the building?
A. KE = GPE
B. KE > GPE
C. KE < GPE
D. It is impossible to tell.

C

If the speed of a car is doubled, the KE of the car is ____________________.
A. quadrupled
B. quartered
C. halved
D. doubled

A

If the speed of a car is halved, the KE of the car is __________________.
A. quadrupled
B. quartered
C. halved
D. doubled

B

Hooke's Law

A law that shows the mathematical relationship between the spring force applied to a spring and the displacement of the string. It can be represented using the equation F = -kx.

Spring Constant

The stiffness of a spring. It is represented using the symbol, “k,” and is measured using the unit, “N/m.” The stiffer the spring, the larger the k.

What does the slope of a straight line is a graph of force as the y-axis and displacement as the x-axis show?

The spring constant of the spring.

Elastic Potential Energy (EPE)

Energy that is stored in a spring by compressing or stretching the spring. The formula used to find this is ½kx².

How do you find the EPE on a graph of force as the y-axis and displacement as the x-axis?

Find the area under the line.

Conservative Forces

Forces that do not change the amount of mechanical energy in a system. This is because they are forces that are internal to the system.

True/False: Force of gravity is an example of a non-conservative force.

False

Non-Conservative Forces

Forces that do change the amount of mechanical energy in a system because they are external to the system.

True/False: Force of friction is an example of a conservative force.

False

Friction

An everyday force that arises from two surfaces interacting.

What type of energy does friction produce?

Thermal Energy

Jerry drops a ball from a height of 2 meters. What type of energy transformation occurs as it falls?
A. KE --> GPE
B. GPE --> KE
C. EPE --> KE
D. EPE --> GPE

B

A marble launcher launches a marble 1.5 meters into the air. As the marble leaves the launcher, what type of energy transformation occurs?
A. KE --> GPE
B. GPE --> KE
C. EPE --> KE
D. EPE --> GPE

C

A marble launcher launches a marble 1.5 meters into the air. At the maximum height, what type of energy transformation has occurred?
A. KE --> GPE
B. GPE --> KE
C. EPE --> KE
D. EPE --> GPE

A

Power

The rate at which work is done. In other words, the rate of energy when transformed. It is measured using the unit, Watts (W). The formula used to find this is W⁄ₜ or Fv.

What types of energy are included in the category mechanical energy?

EPE, KE, and GPE.

What is the only way to change the energy of a system?

When some of the mechanical energy is turned into thermal energy via friction.

What happens to the mechanical energy of the system if no external net force is applied to it?

It is conserved and will remain the same.

If a system is defined as Sita and Jamie, and Sita pushes Jamie, was work done on the system? Why or why not?

Work is not done because the forces done are internal.

If the system is defined as just Jamie, was work done on the system? Why or why not?

Work is done because the forces done are external.

An external force of 50N is applied to an object but the object doesn't move. Did the force do work on the object?

No

If a 50 N force (east) is applied at an angle of 35° above the horizontal instead of parallel to the ground and producing the same displacement of 10 meters east, is there more, less or the same amount of work done? Why?

There is less work being done because the force in the x-direction is less than previously.

If an external force lifts an object to a vertical height of 10 meters, the work done on it causes a change in what type of energy?

GPE

If an external force accelerates of an object, the work done on it causes a chance in what type of energy?

KE

If an external force causes a compression or stretching of a spring (or other stretchy object), the work done on it causes a change in what type of energy?

EPE

A block of mass, m, is pulled over a distance, d, by an applied force, F, which is directed in parallel to the displacement. How much work is done on the block by the force, F?
A. mFd
B. zero
C. Fd
D. F⁄d
E. -Fd

C

A block of mass, m, is moved over a distance, d. An applied force, F, is directed perpendicularly to the block's displacement. How much work is done on the block by the force, F?
A. mFd
B. zero
C. Fd
D. F⁄d
E. -Fd

B

A block of mass, m, is moved over a distance, d. An applied force, F, is opposite to the block's displacement. How much work is done on the block by the force, F?
A. mFd
B. zero
C. Fd
D. F⁄d
E. -Fd

E

A truck driver is trying to push a loaded truck with an applied force. Unfortunately, his attempt was unsuccessful the truck stays stationary no matter how hard the driver pushes. How much work is done by the driver?
A. Fd
B. -Fd
C. F⁄d
D. ᵈ⁄F
E. zero

E

A construction worker holds a heavy toolbox. How much work is done by the worker?
F. Fᵍd
B. -Fᵍd
C. mgh
D. ½mv²
E. zero

E

A spacecraft moves around Earth in a circular orbit with a constant radius. How much work is done by the gravitational force on the spacecraft during one revolution?
F. Fᵍd
B. -Fᵍd
C. mgh
D. ½mv²
E. zero

E

A container with a mass of 5 kg is lifted to a height of 8 meters. How much work is done by the gravitational force?
A. 400 J
B. -400 J
C. zero
D. 50 J
E. -50 J

B

A container with a mass of 5 kg is lifted to a height of 8 meters. How much work is done by the external force?
A. 400 J
B. -400 J
C. zero
D. 50 J
E. -50 J

A

A container with a mass of 5 kg is lifted to a height of 8 meters and then returned to the ground level. How much work is done by the gravitational force?
A. 400 J
B. -400 J
C. zero
D. 50 J
E. -50 J

C

An object is thrown straight up. Which of the following is true about the sign of work done by the gravitational force while the object moves up and then down?
A. Work is positive on the way up, work is positive on the way down.
B. Work is negative on the way up, work is negative on the way down.
C. Work is negative on the way up, work is positive on the way down.
D. Work is positive on the way up, work is negative on the way down.
E. Work is zero the way up, work is zero on the way down.

C

An applied force, F, accelerates an object from rest to a velocity, v. How much work is done by the applied force, F?
A. ½mv²
B. mgh
C. ½kx²
D. mFd
E. zero

A

What happens to the kinetic energy of a moving object if the net work done is positive?
A. The kinetic energy increases.
B. The kinetic energy decreases.
C. The kinetic energy remains the same.
D. The kinetic energy is zero.
E. The kinetic energy becomes negative.

A

What happens to the gravitational potential energy of a moving object if the work done by the gravitational force is negative?
A. The potential energy increases.
B. The potential energy decreases.
C. The potential energy remains the same.
D. The potential energy is zero.
E. The potential energy becomes negative.

A

An object, I, with a mass of 4 kg is lifted vertically 3 meters from the ground level; another object, II, with a mass of 2 kg is lifted 6 meters. Which of the following statements is true?
I. Object, I, has greater potential energy since it is heavier.
II. Object, II, has greater potential energy since it is lifted to a higher position.
III. Both objects have the same potential energy.

A. I
B. II
C. III
D. I and II
E. II and III

C

A bullet penetrates a wooden block and loses its velocity by a half. What is the ratio between the initial kinetic energy of the bullet and kinetic energy when the bullet leaves the block?
A. KEᵢ/KEᶠ = ½
B. KEᵢ/KEᶠ = ¼
C. KEᵢ/KEᶠ = ²⁄₁
D. KEᵢ/KEᶠ = ⁴⁄₁
E. KEᵢ/KEᶠ = ¹⁶⁄₁

D

A truck driver slams on the brakes of a moving truck with a constant velocity, v. As a result of his action, the truck stops after traveling a distance, d. If the driver had been traveling with twice the velocity, what would be the stopping distance compared to the distance in the first trial?
A. Two times greater.
B. Four times greater.
C. The same.
D. Half as much.
E. One-quarter as much.

B

What happens to the total energy of a moving object if all the applied forces are conserved?
A. It increases.
B. It decreases.
C. It remains constant.
D. The velocity is required to answer this question.
E. The altitude is required to answer this question.

C

A block with a mass of m crosses a rough horizontal surface at a constant speed of v. The coefficient of kinetic friction between the block and the surface is µ. How much power must be produced in order to overcome the friction force?
A. mg
B. µmg
C. zero
D. µg
E. µmgv

E