Determine the frictional force

[joelm]

Homework Statement

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.

Homework Equations

[/B]
Coefficient of kinetic force between rubber and dry concrete is 1.0, Fnet = (m)(a) Ff= (u)(Fn)

The Attempt at a Solution

 

[gneill]

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)?

 

[joelm]

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)?

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 that's where to start and if that's correct what would the next step be?

 

[gneill]

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.

 

[joelm]

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.

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

 

[gneill]

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.

 

[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?

 

[joelm]

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?

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).

 

[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?

 

[Cutter Ketch]

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).

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.

 

[gneill]

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.

 

[Cutter Ketch]

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.

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.

 

[haruspex]

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.

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.

 

[gneill]

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.

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.