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Physical Science, 5/e

Energy

# Questions

Note: Neglect all frictional forces in all exercises.

 1 One thousand two hundred joules of work are done while pushing a crate across a level floor for a distance of 1.5 m. What force was used to move the crate? 2 How much work is done by a hammer that exerts a 980.0 N force on a nail, driving it 1.50 cm into a board? 3 A 5.0 kg textbook is raised a distance of 30.0 cm as a student prepares to leave for school. How much work did the student do on the book? 4 A 4.0 hp lawnmower engine moves the mower with a force of 1,492 N over a distance of 100.0 m. How much time did this require? 5 What is the horsepower of a 1,500.0 kg car that can go to the top of a 360.0 m high hill in exactly one minute? 6 (a) What is the kinetic energy of a 30.0 gram bullet that is traveling at 200.0 m/s? (b) What velocity would you have to give a 60.0 gram bullet to give it the same kinetic energy? 7 How much work will be done by a 30.0 gram bullet traveling at 200 m/s? 8 A 10.0 kg box is lifted 15 m above the ground by a construction crane. (a) How much work did the crane do on the box? (b) How much potential energy does the box have relative to the ground? (c) With what velocity would the box strike the ground if it were to fall? 9 A force of 50.0 lb is used to push a box 10.0 ft across a level floor. (a) How much work was done on the box? (b) What is the change of potential energy as a result of this move? 10 (a) How much work is done in raising a 50.0 kg crate a distance of 1.5 m above a storeroom floor? (b) What is the change of potential energy as a result of this move? (c) How much kinetic energy will the crate have as it falls and hits the floor? 11 What is the kinetic energy in J of a 60.0 g tennis ball approaching a tennis racket at 20.0 m/s? 12 (a) What is the kinetic energy of a 1,500.0 kg car with a velocity of 72.0 km/hr? (b) How much work must be done on this car to bring it to a complete stop? 13 The driver of an 800.0 kg car decides to double the speed from 20.0 m/s to 40.0 m/s. What effect would this have on the amount of work required to stop the car, that is, on the kinetic energy of the car? 14 Compare the kinetic energy of an 800.0 kg car moving at 20.0 m/s to the kinetic energy of a 1,600.0 kg car moving at an identical speed. 15 A 175.0 lb hiker is able to ascend a 1,980.0 ft high slope in 1 hour and 45 minutes. (a) How much work did the hiker do? (b) What was the average power output in hp? 16 (a) What distance will a 10.0 hp motor lift a 2,000.0 lb elevator in 30.0 s? (b) What was the average velocity of this elevator during the lift? 17 A ball is dropped from 20.0 ft above the ground. (a) At what height is half of its energy kinetic and half potential? (b) Using energy considerations only, what is the velocity of the ball just as it hits the ground? 18 What is the velocity of a 60.0 kg jogger with a kinetic energy of 1,080.0 J? 19 A small sports car and a pickup truck start coasting down a 10.0 m hill together, side by side. Assuming no friction, what is the velocity of each vehicle at the bottom of the hill? 20 A 70.0 kg student runs up the stairs of a football stadium to a height of 10.0 m above the ground in 10.0 s. (a) What is the power of the student in kW? (b) in hp?