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Homework #20 – Work, Energy, and Power
AP Physics
Problem #1: Conceptual checks …
a. Can a centripetal force ever do work on an object? Explain your answer citing specific examples.
b. Can the normal force on an object ever do work? Explain.
c. A woman swimming upstream is not moving with respect to the shore. Is she doing any work? If she stops swimming and merely
floats, is work done on her?
d. Is the work done by kinetic friction always negative? Consider what happens to dishes on a table cloth when you pull the cloth.
e. Why is it tiring to push hard against a solid wall even though you are doing no work?
Problem #2: A tourist drags his suitcase (weight = 130 N) a distance of 250 meters at a constant speed to catch his flight. He exerts a
60 N force at an angle of 40º above the horizontal.
a. How much work does the tourist do on the suitcase?
b. How much work does friction do against this motion?
c. Find the coefficient of kinetic friction.
Problem #3: A person pushes a stalled 2000 kg car from rest to some final velocity v, doing 4000 Joules of work in the process.
During this time, the car moves 20 meters. Neglect friction.
a. Find the final speed of the car.
b. Find the force the person exerted on the car.
c. Find the acceleration of car while being pushed.
Problem #4: A block is given an initial speed vo up a ramp with an inclined angle θ. The coefficient of kinetic friction between the
block and the ramp is µk.
a. In terms of vo, µk, θ and fundamental constants, find the distance d the box will go up the ramp before coming to rest. Assume the
box never leaves the inclined plane.
b. Given an initial velocity of 3.0 m/s, µ k = 0.5, and θ = 25º, find the distance the box travels.