# Calculating Rolling Friction in a Lab

What would I have to measure to determine the rolling friction of a model car? Currently, I have the horizontal pulling force that the lab procedure calls for on the car, and enough data to make a velocity versus time graph. What else would I need to calculate the rolling friction and how would I do so?

If you assume that the only two forces acting horizontally (when the car is moving) are the pulling force $$F_{\mathrm{p}}$$ and the rolling friction $$F_{\mathrm{f}}$$, then you can use Newton's Second Law, which says that the net force equals the mass times the acceleration. The net force is probably $$F_{\mathrm{p}} - F_{\mathrm{f}}$$ (depending on how you've chosen your signs), and the acceleration is something you should be able to measure reasonably accurately from your graph of velocity versus time (it's the slope). So all you need now is the mass of the car, and then you can solve the equation for $$F_{\mathrm{f}}$$.