Question

It is a ball and incline plane problem with an addition force. I am not sure how to solve it. The link shows the picture and the problem. Thanks.

http://tinypic.com/r/2wgw5zq/6

Answer 1

ok, there is one question I have. Typically with a ball, the coefficient of friction does not matter, the ball rolls, it doesn't slide. I did not answer the last part because the coefficient seems irrelevant. There is no amount of this force that will make the ball go up the ramp. To go up, the force vector must change direction to have a component that is in the negative x direction. Take a look at the diagram I attached, see if it makes sense to you.

Answer 2

egenriether,
Thanks for your respond. I was thinking that the force in the y direction in your diagram would be great enough to keep the ball in its position without rolling down the incline. It is like placing a ball on a vertical wall and press it against it so the force is horizontial, thus the ball wants to drop due to the weight of the ball, but the force pressing the ball against the wall prevents it.
Since that the force is greater in the y direction in the problem you provided, don't you think that the ball will not move? And in order to move it up the incline, it requires a negative 0.707 N force?

Answer 3

recall that the friction force is the normal force times the coefficient of friction. since your mu=1 the normal force from the force (which is just the force itself since the coordinates are along the ramp) must equal the 6.9286N. This would stop it, but it will never allow the ball to go up.

Answer 4

BTW: I mean if the ball is sliding. If it is rolling (typically called rolling without slipping) this means friction is not playing a part and it will roll anyways.

Answer 5

Yes, I believe that you mean that the friction must equal or be greater than 0.707 N and not 6.9286N which is actually 6.9286 m/s2. Thanks for the clearification of the rolling and sliding friction. I don't think the shpere would really change anything if I just wanted it to be changing friction, right?

Answer 6

yes, my bad on the acceleration, its .707N. If you mean that the ball will slide then yes, but if it rolls no friction will stop it because it would not slide along the surface. Thats the reason the wheel was such a good invention. It eliminated sliding friction

Answer 7

Ya, good point on the wheel invention, lol,. Um, so, I guess that even if I have a sphere made of rubber against a concrete ramp, the ball is held in place with theh y value since it is greater than the x value. Like pressing a rubber bouncing ball against the wall. But, if I want to 'roll' the ball up the incline, I will require a force greater than 0.707 N. But, since the incline acts as if a flat surface and easy for a bll to roll, since it will not move with the y values, wouldn't it require little force to roll it up the incline? If not, why?

Answer 8

your force vectors don't add up to give you a component along x in the opposite direction. More of the force vector just pushes it harder onto the ramp surface. If you want it to go up, you'd need to change the angle of F to a vector that has at least some -x component. Then making F big enough would give you a negative x value big enough to equal (and eventually overcome) the positive x value. This is true whether we think the ball rolls or slides. But again if it rolls, we can't stop it (or slow it down) at all. In real life, if you pressed down on a rubber ball you would be touching it which would mean you'd be holding it in place. The force vector here is more abstract, kinda like its weight, where there is no agent actually holding it down, so it will still roll.

Answer 9

I meant if I added a negative x component vector such that it will cause the ball to roll.

Answer 10

yes, that would work then.

Answer 11

So, it will not move when I don't applyy a negative x directional force and it will move if I do. But, it has to be greater than 0.707 N, right? Or it can be smaller, because since the force pressing against the incline makes it not move, as if I rested a ball on a desk. But, i give the ball a small push, requiring very little effort, wouldin't that be like the same situation?

Answer 12

are we talking about the ball rolling or sliding?

Answer 13

When the ball is pressed, it is in the sliding mode. When it is pulled, it moves into the rolling mode.

Answer 14

I don't think that's right. If its to be treated as a ball (like I think) it will always roll. Preventing this requires applying a torque. It will always move unless the new F vector times the sin of the new angle equals 0.707N exactly. Any less and it rolls downhill. Any more and it goes up.

Answer 15

Alright, I am assuming that no matter how much force you apply against a surface on a sphere, it will always roll on an incline. Kind of hard to believe that because the angle of the force and the incline cancles out acting as a flat surface. Thanks for the help.

Answer 16

yes they cancel when you change the vector as we said, but to keep it still requires exactly canceling the weight acting on the incline. It will not roll in that case, but any resultant force (non canceling) will result in ma (I mean newton's law) and it will move. And you're quite welcome!

Answer 17

Or the center of the sphere.