Friction is a force that acts to stop the movement of two touching things. The energy lost to friction is turned into sound and heat. Two kinds of friction are static and kinetic. Static friction is when the friction is strong enough to stop movement between two objects. Kinetic friction is when the frictional force is not strong enough to stop all motion.
We measure kinetic friction, because we are talking about an object moving on a plain surface. With this experiment you can also test the friction on different surfaces, for excample on plastic, on wood, or on different sandpapers. The calculation is a little bit complicated, the formula is below. The 'mp' means pulling weight, 'mm' means moving weight.
μ = - ((mp + mm) * a - (mp * g)) / (mm * g)
Without unit
