Skier and Forces
Science brain teasers require understanding of the physical or biological world and the laws that govern it.
Patrick, a student who was humiliating himself on the ski slopes, decided to take a break in the lodge. Patrick was so discouraged that he turned to his physics teacher for help in skiing. Dr. Moore, the professor, was very keen on seeing daily lives as physics problems. So, the professor wanted Patrick to prove that skiing dealt with conservative forces. The professor said, "My mass is exactly 80kg. If I started from rest at the top of the slope and skied down the slope (total elevation = 110m) and you clocked my speed at 20m/s at the bottom of the slope, would this system be conservative?"
What's the answer to the professor's question?
HintIf this example is conservative, then total initial mechanical energy will equal total final mechanical energy.
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Answer
The system is not conservative.
To solve:
1) First write down the givens:
mass = 80kg
velocity initial = 0
velocity final = 20m/s
height = 110m
2) First calculate the initial potential energy at the top of the slope. Assume that the bottom of the slope is the true point of zero height.
Energy initial = mgh
(80kg)(9.8m/s^2)(110m)=
8.6*10^4J
3) After you found that figure, you must find the mechanical energy at the bottom of the slope.
Energy=1/2mv^2
1/2(80kg)(20m/s)^2=1.6*10^4J
4) Since the final energy is much less than the initial energy, the system is not conservative. Energy was lost on the way down the slope, most likely due to friction and wind resistance.
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Comments
Palsha  
Dec 11, 2002
| Just have a question for everyone. How come everyone is not rating this good? It's a physics question, and those who like physics wouldn't mind trying to find the answer. You don't have to rate problems bad if you don't know how to solve it, or don't know anything about physics. So, if you don't know physics, please don't bother rating my teaser. It's just not logical to rate some of these teasers based on the fact that you can't solve it. |
Bobbrt
Dec 12, 2002
| Palsha, don't take it personally. Some of your teasers will be well-liked and some won't. I like this one, but being an engineer I find it to be pretty simple. Maybe other people who like physics think it is too easy and therefore isn't much fun, or maybe they don't know how to calculate energy and think it's too advanced to be fun. If you look at other teasers' ratings, the results might surprise you. Lots of teasers that I think are terrible have high ratings and some that I love have low ratings. Everybody's opinion is different. Don't get frustrated by it - keep submitting those teasers! |
Palsha
Dec 13, 2002
| Yea, I figured the majority of the people who view these teasers do not have the knowledge of advanced physics yet, so I thought a simple problem would be better for them. Anyone else have any input on my teaser? Any information would be helpful for my next teasers! |
cutebluetiger
Dec 18, 2002
| im one of those people who rated it imbetween boring and fun because i dont know about this stuff, y'all r too brainy! |
jonnyonline
Jan 16, 2003
| This question has been rated poorly because it's boring. The professor's analysis has nothing to do with making the subject a better skier. Generally problems that do well have a "surprise" result or counter-intuitive. |
ghostshadow
Jun 04, 2004
| Yep boring and hard as hell. And in the end your like "hey thats the answer! That was not fun at all!"  |
lelrod
Mar 07, 2006
| I prefer a clever puzzle, not physics homework. I could give you all sorts of homework problems. |
SPUTNIK2 
Feb 07, 2007
| I'not any good at physics |
_numpty_
Jul 03, 2007
| You need to use equations to work this out? Unless there is a frictionless surface mentioned in the physics question it can't be conservative... |
4demo
Dec 24, 2007
| I got it!  We just finished learning this is Physics. Good teaser. |
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