# Determine the frictional force

1. Jan 30, 2017

### joelm

1. The problem statement, all variables and given/known data
If a 2.5 kg rubber ball is rolled across the dry concrete with an acceleration of 4.0 m/s^2[E]. Determine the friction force required to stop the ball.

2. Relevant equations
Coefficient of kinetic force between rubber and dry concrete is 1.0, Fnet = (m)(a) Ff= (u)(Fn)

3. The attempt at a solution

2. Jan 30, 2017

### Staff: Mentor

Hi joelm.

Please note that the rules require that you show an attempt at solution before help can be offered. What have you tried?

Is the problem statement word-for-word as it was given to you? Is the ball really speeding up (the acceleration you stated is positive)?

3. Jan 30, 2017

### joelm

it doesn't say if the speed is consistent or speeding up. I'm confused when they add the acceleration to the question. so I'm assuming you find the net force (2.5 kg)(4.0 m/s^2[E], first I'm not sure if thats where to start and if thats correct what would the next step be?

4. Jan 30, 2017

### Staff: Mentor

As stated, the problem is rather ambiguous. The ball is said to be rolling, so I don't see how kinetic friction would apply or why its coefficient would be given. They don't say why it is accelerating. They don't say if the concrete surface is horizontal or not. They don't say how the ball should come to a stop, whether in some particular timeframe or over a particular distance. So I can't offer any suggestions other than that you find the force as you've done.

5. Jan 30, 2017

### joelm

Yea I'm really confused as well. Would Fnet (Net force) = Normal force? They give you the formula Ff= (u)(Fn) u= coefficient Fn= normal force

6. Jan 30, 2017

### Staff: Mentor

Net force is not normal force. They gave you an equation for the normal force, from which friction force is derived. But as I said, since the ball is rolling (presumably without slipping), kinetic friction doesn't apply. The only other type of friction that might apply is rolling friction, but they didn't give you a formula or coefficient for that.

All you can do with what's given is find the net force that results in the stated acceleration.

7. Jan 30, 2017

### Cutter Ketch

I think what gneill is saying is that you may have mangled the question in your post. It doesn't seem to make much sense. Could you try posting the question again being careful to state it exactly as given?

8. Jan 30, 2017

### joelm

Yes for sure. This is exactly as worded.
If a 2.5 kg rubber ball is rolled across the dry concrete floor with an acceleration of 4.0 m/s^2 [E]. Determine the friction force required to stop the ball. ( Drawing a free body diagram will help illustrate the situation).

9. Jan 30, 2017

### CWatters

I agree with the others - the question is ambiguous.

Friction between what and what? Do they mean friction or rolling resistance? There is a difference.

Is it accelerating because of an applied force or because it's rolling down a slope?

10. Jan 30, 2017

### Cutter Ketch

Well, perhaps it isn't you. At best that question is poorly posed. Anything we suggest here would involve making up an understandable problem that vaguely resembles this. Unfortunately that is not likely to be helpful. I would ask your teacher to clarify.

11. Jan 31, 2017

### Staff: Mentor

Perhaps there was an editing problem in the original text of the problem (is it from a textbook?). If the final sentence was truncated in error and was meant to read:

"Determine the friction force required to stop the ball from slipping."

then you might have a viable question if the coefficient of friction given was for static friction rather than kinetic friction.

12. Jan 31, 2017

### Cutter Ketch

Oh wow! Two words make all the difference! I couldn't think what this problem might mean, but I think you must have nailed it.

13. Jan 31, 2017

### haruspex

Yes, that does sound like the right reading, but I still see a difficulty. To find the friction force we do not need the coefficient of friction. We can find the force from the other information, then find the minimum coefficient.

14. Jan 31, 2017

### Staff: Mentor

True! I suspect that the kinetic friction coefficient that was listed may have been a value looked up in a reference and provided as a potentially relevant value. Perhaps @joelm can confirm/deny this if he returns to the thread.