Download 1. Define the following terms: work kinetic energy mechanical

1. Define the following terms:
work
kinetic energy
mechanical energy
efficiency
energy
reference level
power
potential energy
work-energy theorem
2. When a bowling ball rolls down a level alley, does Earth’s gravity do any work on the ball? Explain.
3. Give a physical example of a machine that could be 80% efficient. Explain what happened to the other 20%.
4. An object slides at constant speed on a frictionless surface. What forces act on the object? What work is
done?
5. Is it possible to get more work out of a machine than you put in? Explain.
6. What is a watt equivalent to in terms of kg, m. and s?
7. Chris carried a carton of milk, weight 10.0 N, along a level hall to the kitchen, a distance of 3.50 m. How
much work did Chris do?
8. Two people of the same mass climb the same flight of stairs. The first person climbs the stairs in 25 s; the
second person takes 35 s.
a) Which person does more work? Explain your answer.
b) Which person produces more power? Explain your answer.
9. Describe the work done and the energy changes taking place when
a) you climb a rope.
b) you throw a ball horizontally.
c) a horizontally thrown ball falls, gaining vertical velocity.
10. A compact car and a semi-truck are both traveling at the same velocity. Which has more kinetic energy?
11. Sally and Lisa have identical compact cars. Sally drives on the freeway with a greater speed than Lisa. Which
car has mor kinetic energy?
12. You and a friend each carry identical boxes to a room one floor above you and down the hall. You choose to
carry it first up the stairs, then down the hall. Your friend carries it down the hall, then up another stairwell.
Who does more work?
13. An airplane passenger carries a 215 N suitcase up stairs, a displacement of 4.20 m vertically and 4.60 m
horizontally.
a) How much work does the passenger do?
b) The same passenger carries the same suitcase back down the same stairs. How much work does the
passenger do now?
14. If two identical bowling balls are raised to the same height, one on Earth and the other on the moon, which has
the larger potential energy relative to the surface of the bodies?
15. How much work does the force of gravity do when a 25 N object falls a distance of 3.5 m? [88 J]
16. An electric motor lifts an elevator that weighs 1.20 x 104 N a distance of 9.00 m in 15.0 s.
a) What is the power of the motor in watts? [7.20 x 103 W]
b) What is the power in kilowatts? [7.20 kW]
17. A rock climber wears a 7.50 kg knapsack while scaling a cliff. After 30.0 min, the climber is 8.2 m above the
starting point.
a) How much work does the climber do on the knapsack? [600 J]
b) If the climber weighs 645 N, how much work does she do lifting herself and the knapsack? [5.9 kJ]
c) What is the power developed by the climber? [3.3 W]
18. An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 m in 35.0 s. How much force
does the motor exert? [1.3 x 105 N]
19. A crane lifts a 2.25 x 103 N bucket containing 1.15 m3 of soil (density = 2.00 x 103 kg/m3 ) to height of 7.50 m.
Calculate the work the crane performs. [1.86x 105 J]
20. Robin pushes a wheelbarrow by exerting a 145 N force horizontally. Robin moves it 60.0 m at a constant
speed for 25.0 s.
a) What power does Robin develop? [348 W]
b) If Robin moves the wheelbarrow twice as fast, how much power is developed? [696 W]
21. A rifle can shoot a 4.20 g bullet at a speed of 965 m/s.
a) Find the kinetic energy of the bullet. [1.96 kJ]
b) What work is done on the bullet if it starts from rest? [1.96 kJ]
c) If the work is done over a distance of 0.75 m, what is the average force on the bullet? [-2.6 kN]
22. A 90 kg rock climber first climbs 45 m upward to the top edge of a quarry, then, from the top, descends 85 m
to the bottom. Find the potential energy of the climber at the edge and at the bottom, using the initial height as
the reference level. [4.0 x 104 J, -3.5 x 104 J]
23. It is not uncommon during the service of a professional tennis player for the racquet to exert an average force
of 150.0 N on the ball. If the ball has a mass of 0.060 kg and is in contact with the strings of the racquet for
0.030 s, what is the kinetic energy of the ball as it leaves the racquet? Assume the ball starts from rest. [170 J]
24. A box that weighs 575 N is lifted a distance of 20.0 m straight up by a rope. the job is done in 10.0 s. What
power is developed in watts and kilowatts? [1.15 x 103 W, 1.15 kW]
25. Pam has a mass of 40.0 kg and she is at rest on smooth, level, frictionless ice. Pam straps on a rocket pack.
The rocket supplies a constant force for 22.0 m and Pam acquires a speed of 62.0 m/s.
a) What is the magnitude of the force? [3.50 x 103 N]
b) What is Pam’s final kinetic energy? [7.69 x 104 J]
Force (N)
Distance (cm)
0.0
0.0
a)Draw a graph to represent the situation.
4.0
2.0
b)Calculate the work done by the man on the shovel over the 6.0 cm. [0.36 J]
8.0
4.0
12.0
6.0
26. A person pushes a shovel into the ground to do some spring gardening.
He applies force to the shovel over the following displacement.
27. Shown is a graph of force versus displacement for an object
being pulled. Determine the work done by the force in pulling the
object 7.0 m.
28. If a power boat puts out 3570 W of power as it moves through the water at 9.2 m/s what is the force of the
water pushing up against the hull of the boast? (hint: draw a FBD, think about how far the boat travels each
second.) [390 N]
29. A 500.0-g bird has gathered a breakfast of 10 juicy worms for her young offspring. Each worm has a mass of
5.00 g and must be delivered to the waiting offspring 10.0 m up in the tree. Calculate the least amount of work
mother bird will have to do to feed breakfast to her little birds. [5.39 x101 J]
30. An elevator motor provides 45.0 kW of power while lifting a 2.00x103 kg elevator 35.0 m. If the elevator
contains six passengers of the same mass and it takes 20.0 s to accomplish this task, determine the mass of
each passenger. [1.03 x102 kg]
Definition of one horse power: