Chapter 5: Work, Power, and Energy
Problems & Exercises
5.1 Work
- How much work does a supermarket checkout attendant do on a can of soup he pushes 0.600 m horizontally with a force of 5.00 N? Express your answer in joules and kilocalories.
Solution: [latex]3\text{.}\text{00}\text{ J}=7\text{.}\text{17}×{\text{10}}^{-4}\text{ kcal}[/latex] - A 75.0-kg person climbs stairs, gaining 2.50 meters in height. Find the work done to accomplish this task.
- (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?
Solution: (a) [latex]5\text{.}\text{92}×{\text{10}}^{5}\phantom{\rule{0.25em}{0ex}}\text{J}[/latex]; (b) [latex]-5\text{.}\text{88}×{\text{10}}^{5}\phantom{\rule{0.25em}{0ex}}\text{J}[/latex]; (c) The net force is zero. - Calculate the work done by an 85.0-kg man who pushes a crate 4.00 m up along a ramp that makes an angle of [latex]\text{20}\text{.}0\text{º}[/latex] with the horizontal. (See figure below) He exerts a force of 500 N on the crate parallel to the ramp and moves at a constant speed. Be certain to include the work he does on the crate and on his body to get up the ramp.
Solution: [latex]3\text{.}\text{14}×{\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}\text{J}[/latex]
- How much work is done by the boy pulling his sister 30.0 m in a wagon as shown in the figure below? Assume no friction acts on the wagon.
- 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 [latex]\text{25}\text{.}0\text{º}[/latex] 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?
Solution: (a) [latex]-\text{700}\phantom{\rule{0.25em}{0ex}}\text{J}[/latex]; (b) 0; (c) 700 J; (d) 38.6 N; (e) 0 - Suppose the ski patrol lowers a rescue sled and victim, having a total mass of 90.0 kg, down a [latex]\text{60}\text{.}0\text{º}[/latex] slope at constant speed, as shown in the figure below. The coefficient of friction between the sled and the snow is 0.100.
(a) How much work is done by friction as the sled moves 30.0 m along the hill?
(b) How much work is done by the rope on the sled in this distance?
(c) What is the work done by the gravitational force on the sled?
(d) What is the total work done?
- *A constant 20-N force pushes a small ball in the direction of the force over a distance of 5.0 m. What is the work done by the force?
- *A toy cart is pulled a distance of 6.0 m in a straight line across the floor. The force pulling the cart has a magnitude of 20 N and is directed at 37°37° above the horizontal. What is the work done by this f