TBR, Physics #20

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Sammy1024

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This is from the shorter set of passages, where there's only 3 passages, for a total with the discretes.

20. A skier is given a strong push so that he slides up the hill in Figure 2 for a certain distance (uk = 0.1). When he gets to the highest point, he slides back down. How does the acceleration of the skier on this ascent compare to the acceleration of the skier on his descent? (Do not consider the acceleration during the initial push).

A. The acceleration on the descent is smaller in magnitude than on the ascent.

Choice A is the best answer. Friction and gravitational force both act on the skier as he goes up the hill and as he slides back down. However, as he moves up the hill, friction and gravity act in the same direction, so these two forces add together. As he slides down the hill, gravity pulls him downward, while friction pulls upward. Now these two forces subtract. The net force acting on the skier as he moves up the hill is greater than the net force acting on the skier as he moves down the hill, so acceleration on the ascent is greater than on the decent.

What I don't understand is if gravity AND friction are working on the skier during the ascent, then how would his acceleration be faster?

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This is from the shorter set of passages, where there's only 3 passages, for a total with the discretes.

20. A skier is given a strong push so that he slides up the hill in Figure 2 for a certain distance (uk = 0.1). When he gets to the highest point, he slides back down. How does the acceleration of the skier on this ascent compare to the acceleration of the skier on his descent? (Do not consider the acceleration during the initial push).

A. The acceleration on the descent is smaller in magnitude than on the ascent.

Choice A is the best answer. Friction and gravitational force both act on the skier as he goes up the hill and as he slides back down. However, as he moves up the hill, friction and gravity act in the same direction, so these two forces add together. As he slides down the hill, gravity pulls him downward, while friction pulls upward. Now these two forces subtract. The net force acting on the skier as he moves up the hill is greater than the net force acting on the skier as he moves down the hill, so acceleration on the ascent is greater than on the decent.

What I don't understand is if gravity AND friction are working on the skier during the ascent, then how would his acceleration be faster?

You should really include page numbers and chapters...

Basically... gravity is always working on the skier to bring him down the hill. This means it always points down the hill. When he is pushed up the hill the friction is also pointing down the hill, because it is opposing his motion. Remember, kinetic friction opposes motion so it points in the direction opposite to the direction of movement. When he is sliding up the hill, the friction opposes this motion and points down the hill, along with gravity.

Just because he got pushed up the hill and is travelling up the hill, does not mean that he does not have an acceleration due to gravity down the hill. Except for the friction, this is like throwing a ball up in the air. The ball is travelling upwards, but it is accelerating downwards because gravity points toward the earth.
 
So the deacceration is a larger magnitude than the acceleration when going down!

Well it isn't so much the magnitude, but rather that both types of deceleration point in the same direction. So the net deceleration downward when the skier is pushed uphill, is greater than the net deceleration downward when he is going downhill.
 
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I'm trying to understand this!

Does kinetic friction always opposes motion? What about when you are ice skating does the friction add to you sliding more (motion)?

Also based on you saying "Remember, kinetic friction opposes motion so it points in the direction opposite to the direction of movement."
When the skier is going up the hill (ascending) there are two forces pointing in the same direction but his motion is opposing both friction and gravity, but when he is descending down the hill mg is same direction as his motion and friction is in the opposite direction, therefore he is only opposing one of the forces so his acceleration should be greater when he is descending then?
 
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