# Finding the Value of the Rolling Friction Force

realslow

## Homework Statement

What is the value of the rolling friction force given 1)Fnet=0.24N (deceleration up the ramp) and 2)Fnet=0.21N (acceleration down the ramp)?

## Homework Equations

Fnet = Fgravity - Ffriction (?)

## The Attempt at a Solution

Somehow I need to combine these values to find the frictional force...
So I tried this:
0.24 = 5.0 - Ffric
0.21 = 5.0 - (-Ffric)
elimination
0.03 = -2Ffric
Ffric = 0.015 (disregarding the negative?)
Does this work?

JaredJames
Are you given the mass of the object in the question? It's not relevant, but you've used a value for it.

Last edited:
realslow
Yes, sorry, the mass was 0.510 kg.

JaredJames
Up Ramp:
Fa = Fgravity + Ffriction
Fgravity = Fa - Ffriction

Down Ramp:
Fb = Fgravity - Ffriction
Fgravity = Fb + Ffriction

Fgravity is equal in both instances so:
Fa - Ffriction = Fb + Ffriction
Fa - Fb = Ffriction + Ffriction
Fa - Fb = 2Ffriction
Ffriction = (Fa - Fb) / 2

Based on that, I'd say I agree with your answer of 0.015.

You don't need the mass of the object for this.

realslow
Yay! Thank you :)
I just wanted to clarify, gravity + friction make up the net force for the cart going up the ramp because gravity and friction are working in the same direction?

JaredJames
Yay! Thank you :)
I just wanted to clarify, gravity + friction make up the net force for the cart going up the ramp because gravity and friction are working in the same direction?

Correct. Both are acting against the direction of motion.

realslow
Okay! :D
Sorry, I just had one question relating to this one, if you could help me again.
I need to state whether my answer is reasonable.
I said it is reasonable because it is 6.8% of the average net force.. and at first I was thinking 0.068 was also the coefficient of friction, but then I calculated friction divided by average net force. It isn't the coefficient.. is it?
I'm also not sure if I was correct in using the average net force.

JaredJames
Okay! :D
Sorry, I just had one question relating to this one, if you could help me again.
I need to state whether my answer is reasonable.
I said it is reasonable because it is 6.8% of the average net force.. and at first I was thinking 0.068 was also the coefficient of friction, but then I calculated friction divided by average net force. It isn't the coefficient.. is it?
I'm also not sure if I was correct in using the average net force.

For this question, assume everything is acting down the slope to avoid any nasty angles in there.

So everything lines up and cancels out nicely.

Yes, you used it correctly.

Is your answer reasonable? Well do you expect the rolling resistance to be high or low?

realslow
Sorry, if you could clarify what you mean when you say, "So everything lines up and cancels out nicely."

I'm not too sure.. I don't think the rolling resistance would be high though.

JaredJames
Sorry, if you could clarify what you mean when you say, "So everything lines up and cancels out nicely."

It's all acting down the slope. Assume net, gravity and friction all refer specifically to those forces acting down the ramp.
I'm not too sure.. I don't think the rolling resistance would be high though.