# Homework Help: Determining the coefficient of static and dynamic friction

1. May 19, 2012

### thestudent101

I have a Mathematics C assignment, with one question being about static and dynamic friction. But I think it fits this forum. Anyway, we need to conduct experiments to show if there is a difference between static and dynamic friction or not with three different surfaces.

The weight of the object being tested is 0.0685kg and gravity has been assumed as -9.8m/s^2

I have calculated the coefficient of static friction for the three surfaces. This was done so by placing an object of weight 0.0685kg on a surface and increasing the angle until the object starts to slide. For wood, the average angle was 37.3°, for glass - 18.0° and for metal - 14.7°. For calculating the static coefficient of friction the weight force has been calculated as
w=mg
w=0.0685*-9.8
w=-0.6713 newtons

This has then been subbed into
0=wsinθ-Fr
0=-0.6713*sin(37.3)-Fr
Fr= 0.407 newtons

0=-wcosθ+N
0=-(-0.6713*cos(37.3))+N
N=0.534 newtons

μ=Fr/N
μ=0.407/0.534
μ=0.762 (wood)
This process has been repeated for glass (μ=0.324) and for metal (μ=0.263)

Now I am completely stuck on how to calculate the coefficient of dynamic friction. I was thinking about timing how long the object takes to slide down a certain length from a set angle? But I don't know how to calculate the coefficient from that? Any ideas and help will be appreciated thanks.

2. May 19, 2012

### Infinitum

To find the co-efficient of dynamic friction, writing the force equations will be a good way to start. Now, if you time how long the object takes to slide down the plane at a certain angle, you will have all the variables to find the acceleration of the object. Substitute this acceleration into the force equation, from which you get the frictional force, and hence, the coefficient.

3. May 30, 2012

### thestudent101

This is due tomorrow, I know I've left it to the last minute but hey.
Anyway I understand how to do that, but how do you work out the acceleration from timing it?

4. May 31, 2012

### dimension10

Re: $s=v_0t+\frac{at^2}{2}$

Use $s=v_0t+\frac{at^2}{2}$