# Homework Help: Rolling Friction

1. Jan 12, 2010

### lovemake1

[Urgent] Rolling Friction

1. The problem statement, all variables and given/known data

How would you calculate a rollin friction? I have a toy car and it is going down the ramp that is 12 degrees above the horizontal.

im very new with this.

what experiment would i have to find this out? please i need to know

2. Relevant equations

3. The attempt at a solution

2. Jan 12, 2010

### cepheid

Staff Emeritus
Re: [Urgent] Rolling Friction

We need more information about your experiment. Did you time how long it took the car to travel down the ramp, and measure how long the ramp is?

Assuming no friction (only gravity), what should the acceleration of the car be? How does this compare to its actual acceleration?

3. Jan 12, 2010

### lovemake1

Re: [Urgent] Rolling Friction

ok yes i have this experiment

** This is a toy experiment ***

the ramp angle = 12 degrees
time taken to travel the ramp = 0.370seconds
length of the plane = 0.1905m
weight of the wheel = 0.3g

the acceleration of the car should be just the x-component of the gravity. but however how would it change with the addition of rolling frictional force??

4. Jan 12, 2010

### cepheid

Staff Emeritus
Re: [Urgent] Rolling Friction

This is motion under constant acceleration. Therefore, if you know the initial velocity, distance travelled, and time taken, you should be able to use basic kinematics to calculate the acceleration (experimental value).

Yes, the component of the weight along the ramp tells you what the acceleration is assuming no friction. If there is friction, then another force acts (in addition to gravity). Remember that Newton's second law says that the acceleration depends upon the NET force. Does that help?

5. Jan 12, 2010

### lovemake1

Re: [Urgent] Rolling Friction

wait.. i've currently developed a formurla for this experiment but instead of adding a
rolling friction i've mistakely added static friction....

ugh.. the whole system of equation right now would be

mgsin(theta) - FR(rolling friction) = ma

but... how do we find FR.. ?? im not sure and vi is 0 for this occasion because i have to realease it.

6. Jan 13, 2010

### cepheid

Staff Emeritus
Re: [Urgent] Rolling Friction

Well, you know mgsin(theta), and you know ma

7. Jan 13, 2010

### lovemake1

Re: [Urgent] Rolling Friction

this is actually my first time dealing with Rollin force, it came to my way unexpectedly...

i searched it and the formula for rolling friction was Fr= CrrNF
how can i use mgsin(theta), and ma to find Fr ?

8. Jan 13, 2010

### cepheid

Staff Emeritus
Re: [Urgent] Rolling Friction

You don't need to know any specific equation for the rolling friction force, you can just calculate what it is. That force balance equation you have written relates those three forces to each other. Since you know two of them, you can solve for the third one.

9. Jan 13, 2010

### lovemake1

Re: [Urgent] Rolling Friction

so it would be

FgSin(theta) - F rolling friction - Ff = ma

- Fr = ma + Ff - FgSin(theta)

this would be correct??

btw how would i be able to calculate the accelerationi without knowing Fr ?
the only way i calculated acceleration in the first place was becuase

Fnet = ma

Fgsin(theta) - Ff = ma

mgsin(theta) - Us mgcos(theta) = ma

mg [ sin(theta) - usCos(theta) = ma

a = g (sin(theta) - usCos(theta))

but now?? how do i calculate for acceleration !!
do i go find velocity using

Last edited: Jan 13, 2010
10. Jan 13, 2010

### cepheid

Staff Emeritus
Re: [Urgent] Rolling Friction

What is Ff? The only thing you can calculate is the total force due to friction (of any kind) by looking at the difference between the theoretical acceleration due to gravity (only) and the actual experimental (measured) acceleration.

11. Jan 13, 2010

### lovemake1

Re: [Urgent] Rolling Friction

huh ! ?