13.5.A.1Multiple Choice
Which statement best defines power?
Select A, B, C, or D to check your answer.
Directions:Work each problem on your own paper first. For multiple choice, select A, B, C, or D. You will see right away whether you are correct. If you miss, you will see the most likely reason, and you can try again. For short response, type your answer, then compare it with the model solution. Questions mix the essential knowledge of Topic 3.5 — power as a rate, average power from energy or work, and instantaneous power from force and velocity. Watch your units: joules are energy, watts are power. Use g = 10 m/s². Round decimals to the nearest hundredth unless noted otherwise.
Which statement best defines power?
Select A, B, C, or D to check your answer.
Two identical 20 kg crates are each raised 2.0 m onto a platform at constant speed. Crate A is raised in 4.0 s and Crate B in 10 s. (a) Find the work done on each crate. (b) Find the power delivered to each crate. (c) Which crate receives energy at the greater rate, and why, if the work is the same?
Engine A and Engine B each do 6000 J of work. Engine A takes 10 s and Engine B takes 20 s. How do their average powers compare?
Select A, B, C, or D to check your answer.
An electric motor transfers 90,000 J of energy in 45 s. (a) Find its average power. (b) How much energy does it transfer in 2.0 minutes at this power? (c) How long does it take to transfer 500,000 J at this power?
A 60 W light bulb is left on for 10 minutes. How much energy does it transfer?
Select A, B, C, or D to check your answer.
An 80 kg hiker climbs 150 m in elevation in 20 minutes. Use the change in gravitational potential energy as the energy transferred. (a) Find ΔUg. (b) Find the hiker's average power. (c) What average power would be needed to make the same climb in 10 minutes?
A 2 kg ball is dropped from rest and falls 45 m in 3.0 s. Ignore air resistance. What is the average rate at which gravitational potential energy is converted to kinetic energy?
Select A, B, C, or D to check your answer.
A crane lifts a 400 kg load 15 m at constant speed in 10 s. (a) Find the work done on the load by the crane. (b) Find the crane's average power. (c) What average power would be needed to lift the same load the same height in 5.0 s?
A person pushes a crate with a constant 50 N force in the direction of its motion for 8.0 m. The push lasts 4.0 s. What is the average power of the person's push?
Select A, B, C, or D to check your answer.
A 1500 kg car accelerates from rest to 24 m/s in 12 s on level ground. (a) Find the net work done on the car. (b) Find the average net power. (c) What average power would be needed to reach 24 m/s in 8.0 s?
Two cars of equal mass start from rest on level ground. Car 1 reaches 10 m/s in 5.0 s, and Car 2 reaches 20 m/s in 5.0 s. What is the ratio of Car 2's average net power to Car 1's?
Select A, B, C, or D to check your answer.
A car cruises at a constant 30 m/s on a level highway. The total resistive force on the car is 800 N. (a) What forward force does the road exert on the car? (b) Find the power the engine delivers. (c) What power is needed at 20 m/s if the resistive force were still 800 N?
A constant 20 N force acts in the direction of motion on an object. The object's speed increases from 2 m/s to 4 m/s. What happens to the instantaneous power delivered by the force?
Select A, B, C, or D to check your answer.
An elevator motor lifts a 1200 kg elevator straight up at a constant 2.5 m/s. (a) Find the force the cable exerts on the elevator. (b) Find the motor's power output. (c) Find the power at a constant speed of 1.0 m/s. (d) How much energy is transferred in 12 s at 2.5 m/s? Check that it equals the gain in gravitational potential energy.
A 40 N force acts at 60° to the velocity of an object that is moving at 5.0 m/s. What is the instantaneous power delivered by the force? (sin 60° = 0.87, cos 60° = 0.50)
Select A, B, C, or D to check your answer.
A constant net forward force of 3000 N accelerates a 1500 kg car from rest on level ground. (a) Find the instantaneous power delivered by the net force when the car is moving at 4.0 m/s. (b) Find it when the car is moving at 12 m/s. (c) Find the time to reach 12 m/s, then find the average power over that time using W/Δt. (a = F/m)
A person carries a box of mass m horizontally at constant speed v. What is the instantaneous power delivered to the box by the person's upward force?
Select A, B, C, or D to check your answer.
The graph shows the total energy transferred to a machine as a function of time. (a) Find the average power from t = 0 to t = 4 s. (b) Find the average power from t = 4 s to t = 8 s. (c) Find the average power over the whole 8 s. (d) Is the power constant?
Which of the following is not a unit of power?
Select A, B, C, or D to check your answer.
Motor A lifts a 100 kg load 6.0 m in 5.0 s. Motor B lifts a 150 kg load 4.0 m in 4.0 s. Both lift at constant speed. (a) Find the average power of Motor A. (b) Find the average power of Motor B. (c) Which motor has the greater power? Compare the work each does.