2/26 HS physics class and homework due 3/4

Hi folks,

Today we took our energy unit up a notch by really getting into the math behind gravitational potential energy, kinetic energy and work.  To bring the math to life, we did a lab where we compared the gPE of a ball to the work done by the ball on a pair of blocks.  We also found the energy efficiency of the system.

For homework answer the following questions.  I've included answers so you can check yourself.  Be sure to really understand these by next week.  Quiz is coming!  Also, look back at that review homework you did a few weeks back.  Next weeks quiz will include some of that material as well.

The following questions are based on today's lab.

Formulas

KE = 1/2mv^2

gPE = mgh

Work = fd

Energy Efficiency = (work out/ work in) x 100  Should always be less than 100)

1. What was the work in, in other words, what type of energy did you put into the system?

2.    What was the work out?  In other words, what did the energy you put in do?

3.  What measurements did you use to find work in and what formula did you use to calculate it?

4.  What measurements did you use to find work out and what formula did you use to calculate it?

5.  What did you do to calculate energy efficiency?

6. In our lab today, what had work done on it?  (If you think carefully, there are two answers here.)

7.  You didn't calculate this but if you think about the pulleys we did two weeks ago, would you say that the pulleys are energy efficient or not?  Why?

8.  We tended to see a trend that the lower the track was the more energy efficient the system was.  Why do you think that's true?

9.  The law of conservation of energy says that energy can neither be created nor destroyed.  But in this experiment work out was less than work in, why?  Where did the energy go?

10.  If there was .5J of potential energy from the ball, what work could have been done on the block if the system was 100% energy efficient?

11.  If there was .5J of potential energy from the ball, how much kinetic energy would the ball have at the bottom of the ramp if the system was 100% efficient?

12.  If there was .5J of PE from the ball and the ball had a mass of .03 Kg, what would the speed of the ball be at the bottom of the ramp (assuming 100% efficiency)?

13.  If there was .5J of PE from the ball how far could the block be moved if the force it takes to move the block is .1N.  (Assume 100% energy efficiency.)

14.  If you doubled the height that you dropped the ball from, what would you expect to see happen to the resulting distance the block was moved?

15. A ski jumper who has a mass of 80 kg stands at the top of a ski jump that is 15m 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 13 m/s, what is his KE as he leaves the ramp?

d.  Hmmmm, his energy in c and his energy in a are different!  He must have lost some energy somewhere.  Where did he lose energy?

e.  What was the energy efficiency of the ramp. (Compare his KE to his PE.)

16. a. If the Irish (little nod to St. Patrick's Day) bobsled with a mass of 320 kg reaches a top speed of 48 m/s, what  energy did the bobsled start with?

b. What height did the bobsled start at?

c. If the stopping area of the track is 30m long, what force does friction apply on our Irish bobsledders.

d. If the height of the track was really 200 m, what was the energy efficiency of the bobsled?   (Compare gPE if the track was 200m tall with the energy you've been working with in this problem.)