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Chapter 7: Q18 CQ (page 259)
Most electrical appliances are rated in watts. Does this rating depend on how long the appliance is on? (When off, it is a zero-watt device.) Explain in terms the definition of power.
No, the rating of the electrical appliances does not depend on how long the appliance is on.
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The 70.0-kg swimmer in Figure 7.44 starts a race with an initial velocity of 1.25 m/s and exerts an average force of 80.0 N backward with his arms during each 1.80 m long stroke.
(a) What is his initial acceleration if water resistance is 45.0 N?
(b) What is the subsequent average resistance force from the water during the 5.00 s it takes him to reach his top velocity of 2.50 m/s?
(c) Discuss whether water resistance seems to increase linearly with velocity.
The person in the figure does work on the lawn mower. Under what conditions would the mower gain energy? Under what conditions would it
lose energy?
A shopper pushes a grocery cart 20.0 m at constant speed on level ground, against a 35.0 N frictional force. He pushes in a direction 25.0º below the horizontal.
(a) What is the work done on the cart by friction?
(b) What is the work done on the cart by the gravitational force?
(c) What is the work done on the cart by the shopper?
(d) Find the force the shopper exerts, using energy considerations.
(e) What is the total work done on the cart?
A toy gun uses a spring with a force constant of \(300{\rm{ N}}/{\rm{m}}\) to propel a \(10.0 - {\rm{g}}\) steel ball. If the spring is compressed \(7.00{\rm{ cm}}\) and friction is negligible:
(a) How much force is needed to compress the spring?
(b) To what maximum height can the ball be shot?
(c) At what angles above the horizontal may a child aim to hit a target \(3.00{\rm{ m}}\) away at the same height as the gun?
(d) What is the gun’s maximum range on level ground?
a) Calculate the work done on a 1500-kg elevator car by its cable to lift it 40.0 m at constant speed, assuming friction averages 100 N. b) What is the work done on the lift by the gravitational force in this process? c) What is the total work done on the lift?
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