Physics
KEY Work, Energy, Power
Name______________________
On a separate sheet of paper, work out solutions to the following problems. Show all work with units.
1. Mr. Letzel claims that he was a hard working bicycle racing star in college, but Mr. Lederhouse says that he was a lazy
bum and didn't deserve any recognition because he hardly "worked" at all.
Explain.
2. If you spit from the second story of a building, it will strike the ground with a certain amount of KE. If you go to the
fourth story, how would its KE of impact compare? How will its velocity compare? Explain.
Double the distance – doubles GPE – doubles KE
3. An 80 kg person carries a 25 N package up a flight of stairs. The vertical height of the stairs is 10.0 meters. How much
work is done? W=Fd = mgd Work is done on person and the package
W= (80 kg +2.5 kg) * 10 m/s2 *10 m =
8250 J
4. A force of 1800 N is needed to keep a piano from sliding down a 10.0 meter ramp. How much work is done?
Zero – no change in position
5. A ball is dropped from a height of 2 meters. It rebounds from the floor with a speed of 3.1 m/s. What % energy was
lost in the collision with the floor?
GPE = KE before
mgd = ½ mv2 -> v2 = 2gd = 2 *10 m/s2 *2 m = 40 …. V= 6.32 m/s before it hits the floor
bounces to .48 m. 76% lost
6. Because there is very little friction, the lever is an extremely efficient simple machine. Using a 90% efficient lever, what
work input is needed to lift an 18 kg mass up a height of .5 meters?
W=Fd =GPE= mgd = 18 kg*10 m/s2 *5 m = 90 J
Efficiency = Wout/Win
.90 = 90J/Win
Win = 100J
100J
Now, for a few more challenging problems.....
7. A arrow of mass .10 kg is fired horizontally from a height of 1.5 m off the ground. The archer exerted an average force
of 350 N on the bowstring to pull it back .70 meters. How far from the archer did the arrow land?
W=Fd = 350N * .7 m = 245 J
KE arrow = 245 J = ½ m v2
So vx2 = 2(245J)/0.1 kg
vx = 70 m/s
Time to fall 1.5 m
D= vit + ½ at2
t2=2d/a = 2(1.5m)/ 10 m/s2
-> t= .55 s
Dx=vxt = 70 m/s*.55s =38.3 m
38.3 m
8. A baseball of mass 140 grams travels at 30 m/s. A fielder's glove moves backward 35 cm during the catch of the ball.
What average force did the fielder exert on the ball?
Need to find the time to bring the ball to a stop
D= vt
T=d/v = .35m/ 15 m/s = .0233 s
FT=mv
F=mv/t = .14 kg * 30 m/s/ .0233 s = 182.6 N
180 N
9. A 24 kg child descends a slide 5 m high and reaches the bottom with a speed of 2.8 m/s. How much thermal energy
from the friction was generated in the child's....bottom?
GPE = mgd = 24 kg*5 m*10 m/s2
= 1200 J
KE bottom = ½ m v2 = ½ 24 kg (2.8m/s)2 = 94.1 J
GPE in = KE bottom + Wfriction
1200 J = 94.1 J + Heat
1106 J
10. A skier traveling at 12.6 m/s reaches the base of a hill that climbs steadily at a 20 degree angle. The skier comes to a
stop after traveling 11.4 meters up along that hill. What is the average coefficient of friction?
KE = =
11. A ball is moving at 2 m/s at position A. Calculate the velocity of the ball at B, C, and D.