# someone help me with a physic question ...

Discussion in 'Physics' started by zelda1850, Jan 3, 2010.

1. ### zelda1850 Thread Starter New Member

Jan 3, 2010
18
0
An 80 kilogram skier slides on waxed skis along a horizontal surface of snow at constant veloctiy while pushing with his poles. what is the horizontal component of the force pushing him forward?

i only know the mass how can i solve this problem?

this was mutiple choice

1) 0.05 N
2) 0.04 N
3) 40 N
4) 4N

2. ### steveb Senior Member

Jul 3, 2008
2,433
469
The calculation is $F=m\; g \; u$ where $m$ is the mass of the skier, $g$ is the acceleration due to gravity (9.807 m/s^2) and $u$ is the coefficient of kinetic friction between the waxed skis and snow. It's simple to calculate if you know $u$ for this case. I don't know what it is; however, I know $u$ for Teflon on Teflon is 4 % (i.e. u=0.04), and I doubt that skis on snow is any better than this. Hence, number 3 appears to be the only answer that seems physically possible to me.

Last edited: Jan 3, 2010
3. ### Fraser_Integration Member

Nov 28, 2009
142
6
you haven't got all the information needed to answer the question.

4. ### steveb Senior Member

Jul 3, 2008
2,433
469
Enough information is given because it is a multiple choice question. Only one answer is reasonable. The teacher or book may have provided approximate coefficients of friction. Even if not, the internet makes tracking down the approximate values very easy.

I was able to answer the question without consulting a reference because I've worked with Teflon and know that Teflon on Teflon has a coefficient of friction of 0.04, and I know that there are very few things more slippery than Teflon on Teflon.

Interestingly, an example of something much more slippery can be found in our own joints. See the following table.

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May 11, 2009
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6. ### steveb Senior Member

Jul 3, 2008
2,433
469
How did you get 23 Newton? The data you quoted shows the coefficient of kinetic friction is 0.05 for waxed skis on snow.

Hence, F=80*9.807*0.05=39.23 Newtons.

So now we have completely answered the question and the OP need do absolutely no work at all. Congratulations to us !

7. ### Fraser_Integration Member

Nov 28, 2009
142
6

Well my point still stands, not enough information was given, and I wouldn't have thought you'd have access to Google in an exam.

8. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,224
Well, yeah... Huh, of course i did know that. But thanks anyway. I was just checking if the OP was paying attention

9. ### steveb Senior Member

Jul 3, 2008
2,433
469
If your point still stands, it does so with crutches. The question may not be worded perfectly, but the intent is to see if the student has any feel at all for orders of magnitude and whether answers are physically reasonable.

Who said this is an exam question? This is listed as a homework question, so the idea is to figure out the answer and not give up. If it were an exam question, then obviously the teacher would be testing knowledge of reasonable range of coefficient of friction in a real world problem.

One can always claim that a question does not provide all the information if we assume the reader has an empty mind and no motivation to think or study. A minimum knowledge level of the student is always assumed.

In the old days we had to go to the library to look up and learn this kind of information. Now it's available in 30 seconds with a few key strokes. How easy does a question need to be before you have "all the information needed to answer the question"?

Last edited: Jan 4, 2010