1. A weightlifter is raising a 100 kg barbell above his head at constant velocity. What force is he applying to the barbell?

Transcription

1 1. A weightlifter is raising a 100 kg barbell above his head at constant velocity. What force is he applying to the barbell? 2. A weightlifter is holding a 100 kg barbell above his head. What force is he applying to the barbell? Work Energy is the ability to do work, but what is work? 1

2 3. A force of 14 N is applied to a 2 kg mass over a distance of 6 meters. What work is done on the mass? 4. Ten Joules of work are used to push a box for 2 meters. What force was applied to the box? 5. A weightlifter lifts a 100 kg barbell 2 m above the ground. a. What force did he overcome to lift the barbell? b. How much work did he do? How does a lever work? 6. How much work is required to lift a 60 N object 2 meters? Would it be possible to do the same work by only applying a 20 N force? How? This is how levers and pulleys allow people to lift heavier loads than they would normally be able to. 2

3 Work at an angle 7. A little boy pulls a wagon with a force of 10 N at an angle of 30 degrees from the horizontal. If he pulls for 20 meters, how much work does he do? 8. A 10 kg crate is pulled 20 m across the floor by a rope making a 25 degree angle with the floor. A force of 200 N is applied to the rope and it takes 10 seconds for the crate to move across the floor. How much work is done? 9. As shown in the diagram, a 2 kg box is lifted up the stairs from point A to point B. a. In what direction is the force one must apply to lift the box? b. Which distance do you think you use? Calculate the work in lifting the box. 3

4 10. A 2 kg book is lifted from the floor to the top of a 1.2 meter tall table. What is the change in energy of the book? 11. A student does 60 J of work to lift a 20 N box. Through what distance did he lift the box? 12. Work is being done when a force: (1) acts vertically on a cart that can only move horizontally (2) is exerted by one team in a tug of war when there is no movement (3) is exerted while pulling a wagon up a hill (4) of gravitational attraction acts on a person standing on the surface of the Earth 13. An ant carries a 0.05 kg pebble up to the 10 th floor, 30 m above the ground. a. How much work does the ant do? b. How high would you have to lift a 0.8 kg textbook to do the same work? 4

5 Power Home Entertainment Due tomorrow Text p #1,2,18 Test Monday Circular Motion, Universal Gravitation, Work & Power Due Monday 2/2 Timer Tape Lab 14. A toy car is pushed at a constant velocity by a force of 6 N across a 9 meter floor. The car crosses this distance in 3 seconds. How much power was used to push the car? 15. A motor is capable of performing 200 J of work every 12 seconds. What power does it generate? 16. A boat weighing 900 N requires a horizontal force of 600 N to move it across the water at 15 m/s. How much power must be provided by the boat s engine? 5

6 17. A 3 kg block is initially at rest on a frictionless horizontal surface. The block is moved 8 meters in 2 seconds by the application of a 12 newtons. What is the average power developed while moving the block? 18. A 1500 W engine lifts a box for 30 seconds. How much work did the engine do? 19. What amount of force must be applied to an object if 200 J of work is done moving it 0.5 m? 20. A 70 kg cyclist develops 210 watts of power while pedaling at a constant velocity of 7 meters per second. What average force is exerted eastward on the bicycle to maintain this constant speed? 21. How much work is done on a skier who applies an average braking force of 9.8 x 10 2 N and comes to a stop in 10 meters? 22. A student does 60 joules of work pushing a 3 kilogram box up the length of a ramp that is 5 meters long. What is the magnitude of the force applied to the box to do this work? 6

7 23. A 5 kg wooden block slides 12 meters across a wooden surface a. Calculate the force of friction acting on the block b. Calculate the amount of work done by friction as the block crosses the surface 24. A 70 N steel box is dragged across a steel floor at 15 m/s. a. Calculate the force of friction acting on the box b. Calculate the power required to pull this box. 25. How much work can be completed by a 1200 watt motor in 5 minutes? 26. A small electric motor is used to lift a 0.50 kg mass at constant speed. If the mass is lifted a vertical distance of 1.5 meters in 5 seconds, what is the average power developed by the motor? 27. How much work is required to lift a 10 newton weight from 4.0 meters to 40 meters above the surface of Earth? 31. A truck weighing newtons was driven up a hill that is meters long to a level area that is meters above the starting point. If the trip took 480 seconds, what was the minimum power required? 13. A jack exerts a force of 4,500 N to raise a car by 0.25 m. What is the work done by the jack? 14. A garden tractor pulls a plow at an angle of 25 degrees. If it pulls with a force of 500 N a distance of 10 m in 20 seconds, how much work is done? 7

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