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I will use the example of a golf ball here, but there are many other examples I guess.

My question is:

A golf ball is at rest on an incline that slopes from right to left. I take it the force from friction is the same as the gravitational force so it stays in place. Now I apply force to the ball and roll it forward along the slope. Now the ball starts moving from right to left down the incline.

Why, when the surface (friction?) is constant, and the only vector applied is forward, does the ball now start to move from right to left down the slope?

Why does a forward force applied, cause the ball to move sideways, why is the friction now suddenly not enough to keep it going straight?

If that makes sense!

sammy gerbil
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user1525612
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  • Static vs Kinetic coefficient of friction. Also, grass is not a smooth surface and a golf ball is not a perfect sphere so as it rolls all it needs is to find divet down to the left and then it has momentum that way and can continue down that path. – Ulthran Aug 16 '16 at 17:48
  • If the golf ball were a perfectly rigid sphere and the incline were perfectly rigid as well, there'd be no way to have it sit stationary. – Kyle Aug 16 '16 at 17:57

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I take it the force from friction is the same as the gravitational force so it stays in place.

This is too simplistic and overloads the term "friction". Better to say "rolling resistance" here. What is the cause of rolling friction? & why is it less than sliding friction?

A spherical ball sitting still on a slope must be creating an indentation. This indentation allows normal forces to point in a direction that hold it in place.

Why, when the surface (friction?) is constant, and the only vector applied is forward, does the ball now start to move from right to left down the slope?

Because the specific shape of the indentation changes as the ball rolls, the forces on it are not constant. Even true static friction forces may not be constant. Once you push the ball out of its spot, you change the shape of the indentation. Some of the normal forces now push the ball so that it rolls in the down-slope direction.

Community
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BowlOfRed
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If the slope had been perfectly flat, the ball would have rolled down it when you placed it on the ground. It would not have stayed where you put it. Gravity would have pulled it down the slope. If there were no friction, the ball would slide down the slope. But because there is friction between the ball and the slope, even when the slope is perfectly flat, it rolls down instead.

The reason the ball does not roll downhill when you place it on the ground is not because of friction but because the ground is not flat enough. Small obstacles which prevent it from doing so - eg it might have to roll over some grass, which requires force to lift it up a little. Because it is not already moving, it has no momentum or inertia to overcome the resistance provided by the grass.

When you apply a force (or rather an impulse) by putting the ball, it rolls forward along the slope. It keeps rolling after it has left contact with the club because it has momentum or inertia.

As it rolls forward, because the ground is a little rough, the ball bounces slightly, because it hits small obstacles. Bouncing enables the ball to overcome other small obstacles not only in front of it but also to the side. Gravity is still trying to pull the ball downhill but now that the ball is bouncing a little it can overcome the obstacles (ie grass) to the side also, and starts to move sideways.

If it builds up enough momentum rolling to the side it will continue rolling downhill even after it has stopped rolling along the slope.

sammy gerbil
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