# Applications of Newton's laws - coefficient of friction

• Jujubee37
That is a difficult question to answer. I think it would depend on the specifics of the situation. If the coefficient of static friction is greater than the coefficient of kinetic friction, then it probably slid. If the coefficient of static friction is less than the coefficient of kinetic friction, then it probably doesn't slide.f

#### Jujubee37

Homework Statement
A skier traveling 12.9-m/s reaches the slope of a steady upward 14.9-degree incline and glides up 13.4-m up this slope before coming to rest. What was the average coefficient of friction between the skies and the slope?
Relevant Equations
fr = Fr/N is an equation to use but im not sure how that equation would work in this case. The answer should be a value between 0-1 but I keep getting something over that.
https://www.physicsforums.com/attachments/277763

Please show us the answer you got and the math that produced it. Then we will be able to diagnose if and where you went wrong.

0=12.9^2+2(a)(13.4) which got me -6.2 m/s/s

0=12.9^2+2(a)(13.4) which got me -6.2 m/s/s
That would be the acceleration if the skier were on a horizontal surface. Here the skier is going up a hill and gravity also has something to say about the acceleration. Draw a free body diagram of the skier.

• Jujubee37
okay so I did the diagram and worked out the equation. 9.8sin(14.9)+u(9.8)cos(14.9)=6.2 which got me 0.388 which is the correct answer. thank you

• kuruman
okay so I did the diagram and worked out the equation. 9.8sin(14.9)+u(9.8)cos(14.9)=6.2 which got me 0.388 which is the correct answer. thank you
You are welcome and also welcome to PF. • Jujubee37
The answer should be a value between 0-1
There's no upper limit to coefficients of friction, certainly nothing special about a value of 1.
But for a skier on snow, you would expect it to be rather lower.

There's no upper limit to coefficients of friction, certainly nothing special about a value of 1.
But for a skier on snow, you would expect it to be rather lower.
That is my understanding too. Nevertheless, out of curiosity I visited several sites looking for coefficients of kinetic friction greater than 1. The only one I could find was aluminum on aluminum with μs = 1.05-1.35 and μk = 1.4. This is not a typographical error on my part. See e.g. here. I encountered the same numbers elsewhere which makes me wonder whether they copy from each other trusting that the initial report of these numbers is correct.

The question of why the coefficient of static friction is (generally) greater than the coefficient of kinetic friction has appeared on many PF threads of which I list only the first four:
https://www.physicsforums.com/threa...ways-smaller-than-the-static-friction.140426/

Is aluminum a friction anomaly? I do not know. However, I do know that although aluminum skis exist, aluminum ski slopes do not.

That is my understanding too. Nevertheless, out of curiosity I visited several sites looking for coefficients of kinetic friction greater than 1. The only one I could find was aluminum on aluminum with μs = 1.05-1.35 and μk = 1.4. This is not a typographical error on my part. See e.g. here. I encountered the same numbers elsewhere which makes me wonder whether they copy from each other trusting that the initial report of these numbers is correct.

The question of why the coefficient of static friction is (generally) greater than the coefficient of kinetic friction has appeared on many PF threads of which I list only the first four:
https://www.physicsforums.com/threa...ways-smaller-than-the-static-friction.140426/
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