3/4 Physics update and homework due 3/18

Hi folks,

Remember no class next week due to spring break.  I'll see you all again on 3/18.

Today we did a bit more work with the relationships between kinetic energy, potential energy and work.  We also did a fairly complex lab where we had to use our formulas and ingenuity to calculate the energy efficiency of a ramp and ball system. 

Here's problem #8 on the quiz that we didn't have time for today.  It would be a great idea to be prepared for this on a future quiz.

8. You and grandma are having a food fight. She hurls a 156g pile of potatoes at you at 15m/s. At the same time you throw a 58g tomato at her at 19m/s. As luck would have it they
collide in midair and stick together. What is the final speed of the tomato/potato missile? 

For homework due 3/18 please do the following problems.

1. Thinking about the energy formulas and the "roll the vehicle down the ramp" activity we did last week,  tell me if it make sense to have a heavy vehicle or a light vehicle as far as gPE goes.

2. If you created a balloon car, would you want it to be heavy? Why or why not?

3. If you and a friend were on identical sleds and your friend weighs twice as much as you do, which of you would go farther? Which of you would have more velocity? Describe this in relation to momentum and describe this in relation to energy.


Use these formulas to solve the following Olympic energy problems. Answers below.
gPE = MgH
KE = ½ MV^2 (^2 means squared)
W = FD

1. A ski jumper who has a mass of 70 kg stands at the top of a ski jump that is 20m tall.

a. What is his potential energy?


b. If there's no friction, what would his speed be as he launches from the jump?



c. If his speed is 14 m/s, what is the energy efficiency of the ramp?


d. What work did friction do on our ski hero?


e. What was the average force of friction between the ramp and his skis if he travels
75 m down the ramp


2. a. If the Jamaican bob sled with a mass of 300 kg reaches a top speed of 150 km/h (42 m/s), what was the least amount of energy the bob sled started with?

b. What was the lowest height the bob sled started at?

c. If the track is 2200m long, what work did friction do on our Jamaican bob sledders by the time the team comes to a complete rest at the bottom?

d. If the height of the track was really 200 m, what was the energy efficiency of the bob sled?




1. a. gPE = Mgh
70kg x 20 m x 10 m/s^2 = 14,000J

b. KE = ½ MV^2
14,000 J = ½ 70 kg x v^2
v = 20 m/s

c. KE = ½ 70kg x (14)^2
KE = 6860 J
Work out/ Work in x 100
6860/14000 x 100 = 49% Efficient

d. 14,000J - 6860J = 7140 J

e. W = FD
7140J = F x 75m
F = 95.2 N



2. a. KE = ½ MV^2
KE = ½ 300kg (42 m/s)^2
KE = 264,600 J

b. gPE = MgH
264,600 = 300kg x 10m/s^2 x H
H = 88.2 m

c. W = FD
264,600 J = F x 2200 m
F = 120.27 N

d. gPE = MgH
gPE = 300 kg x 10 m/s^2 x 200 m
gPE = 600,000 J That's the maximum amount of energy. Since their energy was 264,600 J their energy efficiency was…
Work out / Work in x 100
264,600/ 600,000 x 100 = 44%