# Calculation of Friction

• B

## Main Question or Discussion Point

So Ive done a practical investigation.
Pretty much I used a elastic band to launch a puck on a horizontal floor. I have measured the extension and force for the elastic band, and the extension and stopping distance for the puck on two different surfaces.
How do I calculate the coefficient of friction?
thanks.

Related Classical Physics News on Phys.org
BvU
Homework Helper
2019 Award
How do I calculate the coefficient of friction?
Actually, there may be two of those: one as long as it's not moving and one if it is (the latter is usually smaller).
Did you calibrate your rubber band ? Is it a bit linear (like an ideal spring: extension = k $\times$ force) ?

Are you familiar with friction coefficient formulas like $F_{\rm friction} = \mu F_{\rm normal} = \mu\, mg$ ?

Yeah, it's linear.

I have a force vs ext for the band, but dont have the force applied when I used it for the puck being launched.

You must calibrate the band for obtain k.

Then you need to suposse that all elastic energy of the band is transferred to the puck in the form of kinetic energy. Then: $$\frac 1 2 k·x^2= \frac 1 2 m·v_0^2$$

Then you can calculate the aceleration $$0=v_0^2+2·a·distance$$

During the mouvement there are only friction force, then: $$\mu·m·g=m·a$$
(mass cancel)
I don't know if there are some wrong in this procedure, please say me if this procedure is not correct. Sorry for my English.

• Even in a moore compact way: Eband=Wfriction $$\frac 1 2 k·x^2=\mu·m·g·distance$$

• Thanks guys!

sophiecentaur
• 