1. Chapter 5
Work and Energy

2. Question
A crate of mass 10 kg is on a ramp that is inclined at an angle of 30⁰ from the horizontal. A force with a magnitude of 135 N directed parallel to the ramp is used to pull the crate with a constant speed up the ramp a distance of 6 m. What is the work done on the crate by the frictional force?

3. Answer
W = -516 J
It is negative because friction acts in the exact opposite direction as the displacement
Figure out the parallel component of the weight
Use the net force (set equal to zero) in the parallel direction to find the frictional force
Use W=Fd where F is the frictional force to find out the work done by friction

4. Question
An empty coffee mug with a mass of 0.40 kg gets knocked off a tabletop 0.75 m above the floor onto the seat of a chair 0.45 m above the floor. Assume that the gravitational potential energy is measured using the floor as the zero reference level. What is the change in gravitational potential energy for the mug?

5. Answer
∆GPE = J
Answer is negative because final gravitational potential energy is less than initial

6. Question
A pendulum swings back and forth. If you take its lowest point as your zero reference level, where would its kinetic energy be greatest? Where would its GPE be greatest?

7. Answer KE is greatest at lowest point of swing (zero reference level)
GPE is greatest a highest points of swing
Greatest GPE
Greatest GPE
Greatest KE

8. Question
A satellite is in a circular orbit above Earth’s surface. There is no work done on the satellite because the force of gravity is always perpendicular to the satellite’s motion. Based on this, what does the work-kinetic energy theorem predict about it’s speed?

9. Answer
The speed is constant

10. Question
Advertisements for a toy ball once stated that it would rebound to a height greater than the height from which it was dropped. Is this possible?

11. Answer
No, because energy would not be conserved

12. Question
A student walks up a staircase on their way to class and then runs down it on their way to lunch. In which situation do they exert more power?

13. Answer
Running down it
The same amount of work is done going up and down the staircase; however, running decreases the amount of time it takes to do that work therefore increasing the power exerted

14. Question
If the speed of a car is doubled and the mass is tripled, by what factor does the kinetic energy of the car increase?

15. Answer 12
KE = 1/2mv2 changes to KE = ½(3m)(2v)2 = ½(3m)(4v2) and 4x3 = 12

16. Question
A 70 kg base runner moving at a speed of 4.0 m/s begins to slide into second base. The coefficient of friction between his clothes and Earth is His slide lowers his speed to zero just as he reaches the base. How far does the runner slide?

17. Answer
0.3 m
Use the work kinetic energy theorem; the force doing the work is friction

18. Question
A spring scale has a spring with a force constant of 250 N/m and a weighing pan with a mass of kg. During one weighing, the spring is stretched a distance of 12 cm from equilibrium. During a second weighing, the spring is stretched a distance of 18 cm. How much greater is the elastic potential energy of the stretched spring during the second weighing than during the first weighing?

19. Answer 2.25 times greater Don’t forget to convert cm to m
Remember “x” is the difference between the relaxed length and the stretched length

20. Question
A student with a mass of 66 kg climbs a staircase in 44 s. If the distance between the base and the top of the staircase is 14.0 m, how much power will the student deliver by climbing the stairs?

21. Answer
206 W
The force that the student exerts in climbing the stairs that does work is exactly equal and opposite to the student’s weight

22. Question
A 75 kg man jumps from a window that is 1 m high. Calculate the man’s speed when he strikes the ground.

23. Answer
4.4 m/s
Use conservation of energy equation to solve

24. Question
A sled is pulled along a flat surface with a force of 17 N directed at an angle of 25⁰ above the horizontal. If the sled is pulled a distance of 3 m, how much work is being done?

25. Answer
46.2 J

26. Question
There are 2 situations that others, not as enlightened as you, may be inclined to say that “no work is being done”. Name them.

27. Answer
When a force is applied but there is not resulting displacement (ex: you push on a car but it doesn’t move)
When the force is applied perpendicular to the displacement (ex: a waitress carrying a tray)

28. Question
If I am looking to find the net work done on an object (as required when using the work-kinetic energy theorem), what force must be used in the equation W = Fd?

29. Answer
The net force

30. Question
An object slides down an incline and it is stated that mechanical energy is conserved in this situation. What is being ignored in that statement?

31. Answer
Friction
When friction is present, mechanical energy is NOT conserved

32. Question
What form of the conservation of energy equation would you use to solve a problem involving a roller coaster climbing a hill?

33. Answer
KEi = GPEf

34. Question
Are work and energy scalar or vector quantities?

35. Answer
Both are scalar

36. Question
For conservation of energy, what type of energy is ALWAYS conserved?

37. Answer
Total
The answer is NOT mechanical because in the presence of friction, mechanical energy is NOT conserved

38. Question
How is an object’s kinetic energy affected by friction?

39. Answer
Decreased
Friction slows things down; KE is based upon speed

40. Question
There are 2 springs. A 1 kg mass is hung from each spring. As a result, spring 1 stretches 7 cm and spring 2 stretches 2 cm. Which spring has a greater spring constant, k?

41. Answer
Spring 2
Stiffer springs have larger k and therefore will stretch/compress less when subjected to a force

42. Question
Bud, a very large man of mass 130 kg, stands on a pogo stick. How much work is done as Bud compresses the spring of the pogo stick 0.50 m?

43. Answer J

44. Question
Frank, a San Francisco hot dog vendor, has fallen asleep on the job. When an earthquake strikes, his 300 kg hot dog cart rolls down Nob Hill and reaches point A at a speed of 8 m/s. How fast is the hot dog cart going at point B when Frank finally wakes up and starts to run after it? A B
50.0 m
30.0 m

45. Answer
19.8 m/s

46. Question
On a ski weekend in Colorado, Bob, whose mass is 75.0 kg, skis down a hill that is inclined at an angle of 15.0° to the horizontal and has a vertical rise of 25.0 m. How much work is done by gravity on Bob as he goes down the hill?

47. Answer
18,391.2 J

48. Question
At an amusement park, a ride called “Cyclone” is a giant roller coaster that ascends a 34.1 m hill and then drops 21.9 m before ascending the next hill. The train of cars has a mass of 4727 kg. What power must be generated to bring the train to the top of the first hill in 30.0 s?

49. Answer
52,709.4 W