AP Physics 1: Algebra-Based, AP exams, 18–23%

Unit 3: Work, Energy, and Power

Unit 3 shares the heaviest published weighting with Unit 2. It defines work as an energy transfer, develops kinetic and potential energy, and uses conservation of energy as an alternative to force analysis. Choosing between an energy approach and a dynamics approach is itself a tested skill.

Position: 3 of 8 units
Published weighting: 18–23% in the official framework
Topics covered: 5 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Physics 1: Algebra-Based. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Translational kinetic energy: The dependence on mass and the square of speed, and the work-energy relationship that changes it.
Work: Work as force through a displacement, the role of the angle between them, and work from a force-versus-position graph.
Potential energy: Gravitational and elastic potential energy, the arbitrariness of the reference level, and energy diagrams.
Conservation of energy: Tracking energy through a system, accounting for dissipation, and knowing when the total is conserved.
Power: Power as the rate of energy transfer, and the relationship between power, force, and speed.

Study tips

How to practice Work and Energy.

Section-specific guidance that follows from how this material is usually tested.

Define the system and its boundary before writing an energy equation; most errors here come from an unclear system.
Practice deciding between energy and force methods on the same problem to see when each is faster.
Use energy bar charts for multi-stage problems so nothing is dropped between stages.

Where this fits

Unit 3: Work, Energy, and Power is 3 of 8 units in the AP Physics 1: Algebra-Based guide, and it carries a published weighting of 18–23% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Physics 1: Algebra-Based guide Official exam information

Sample Work and Energy questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
A rope pulls a crate 8.0 m across a level floor while applying a constant 50 N force directed 60 degrees above the horizontal. How much work does the rope do on the crate?
1. A50 J
2. B200 J
3. C346 J
4. D400 J
Show answer
Answer: B. 200 J
Work is W = Fd cos(theta) = (50 N)(8.0 m)cos(60 degrees) = (50)(8.0)(0.50) = 200 J. The 400 J choice ignores the angle entirely, and 346 J uses sin(60 degrees), which would be the component perpendicular to the displacement and does no work.
Key insight: Only the component of force along the displacement does work, so a force perpendicular to the motion does none.
Question 2Foundational
A 60 kg student climbs a staircase of vertical height 4.0 m in 8.0 s at a steady pace. Using g = 10 m/s^2, what average power does the student develop against gravity?
1. A30 W
2. B150 W
3. C300 W
4. D2400 W
Show answer
Answer: C. 300 W
The work done against gravity is mgh = (60 kg)(10 m/s^2)(4.0 m) = 2400 J, and average power is that work divided by time: 2400 J / 8.0 s = 300 W. The 2400 W choice reports the work as though it were power, and 30 W leaves g out of the energy calculation.
Key insight: Power is the rate of energy transfer, so the same climb done in half the time requires twice the power.
Question 3Intermediate
A 0.50 kg cart slides at 4.0 m/s along a frictionless track and runs into a spring of spring constant 200 N/m, compressing it until the cart stops momentarily. What is the maximum compression of the spring?
1. A0.040 m
2. B0.10 m
3. C0.20 m
4. D0.40 m
Show answer
Answer: C. 0.20 m
All of the kinetic energy becomes elastic potential energy: (1/2)(0.50)(4.0)^2 = 4.0 J and (1/2)(200)x^2 = 4.0 J gives x^2 = 0.040 m^2, so x = 0.20 m. The 0.040 m choice reports x^2 without taking the square root.
Key insight: Elastic potential energy depends on the square of the compression, so doubling the incoming speed doubles the compression.
Question 4Intermediate
A block is released from rest and slides down a frictionless ramp through a vertical drop of 1.8 m. Using g = 10 m/s^2, what is the speed of the block at the bottom of the ramp?
1. A3.0 m/s
2. B6.0 m/s
3. C18 m/s
4. D36 m/s
Show answer
Answer: B. 6.0 m/s
Setting mgh equal to (1/2)mv^2 cancels the mass and gives v = sqrt(2gh) = sqrt(2 x 10 x 1.8) = sqrt(36) = 6.0 m/s. The 36 m/s choice reports 2gh without taking the square root.
Key insight: On a frictionless ramp the final speed depends only on the vertical drop, never on the mass or the ramp angle.
Question 5Advanced
A 2.0 kg box starts from rest and is pushed 5.0 m across a level floor by a constant 30 N horizontal force while a 10 N friction force opposes the motion. What is the speed of the box after those 5.0 m?
1. A5.0 m/s
2. B10 m/s
3. C12 m/s
4. D14 m/s
Show answer
Answer: B. 10 m/s
The net work is (30 N - 10 N)(5.0 m) = 100 J, which equals the change in kinetic energy, so (1/2)(2.0)v^2 = 100 J and v = 10 m/s. The 12 m/s choice uses the applied force alone and ignores the energy dissipated by friction.
Key insight: The work-energy theorem uses the net work, so every force acting through the displacement must be included.

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Unit 3: Work, Energy, and Power