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AP Physics 1 · Unit 3 · Energy

Power Practice Problems

Compare how quickly work is done and energy is transferred. Practice average and instantaneous power in lifts, accelerating motors, braking, angled pulls, and a changing force graph.

12 questions6 conceptual · 6 calculation

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Simulation scene from the Power lesson.Open interactive practice
A scene from the related lesson. Open the practice to work through the questions.

What you’ll practice

Connect the energy transfer with its rate

Compare work and average power

Find work over a time interval and explain how completing the same lift sooner changes average power. Compare two accelerating motor trials.

Calculate instantaneous power

Use the component of force along velocity. Distinguish a motor’s power from net power and determine when a brake’s power is negative.

Combine power with other measurements

Calculate useful motor efficiency and use a force–position graph to connect work, speed, and power at selected positions.

A question from this set

The same lift in different times

Two motors lift identical loads through the same vertical height at constant speed. One completes the lift in half the time. How do the useful work and average mechanical power compare?

  • The faster lift requires twice the work and twice the average power.
  • The work is the same, and the faster lift has twice the average power.
  • Both work and average power are the same.
Try it in the practice set
The same lift in different timesTwo motors lift identical loads through the same vertical height at constant speed. One completes the lift in half the time. How do the useful work and average mechanical power compare?LoadLoadTime TTime T / 2Same load · same lifting height
Identical loads rise through the same height in different times.

Key ideas

Average and instantaneous power describe different questions

Average power is work divided by elapsed time

For a specified interval, Pavg = W/Δt. Two motors can do the same useful work but have different average powers because their completion times differ.

Instantaneous power uses current velocity

For one force, P = Fv cos θ. With a constant force parallel to an accelerating cart, power increases as the cart speeds up. A perpendicular force delivers zero mechanical power.

See the physics

Compare the same lift at two speeds

Check your response, then switch motor trials and inspect height, work, elapsed time, and power. Other questions compare braking and accelerating motion with the power at a particular instant.

Use a hint for a starting point or open a worked solution to follow the reasoning. Your progress saves in this browser, so you can return to unfinished questions.

Ready for power?

Begin with a conceptual question, then work through the calculations.

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