Determining the Coefficient of Kinetic Friction
In the setup we used to verify Newtonโs second law, suppose that we take into account the effect
of kinetic friction. Without kinetic friction the acceleration of the cart sliding on the horizontal
track and pulled by the hanging mass is given by Newtonโs second law:
๎๎ + ๎
๎
๎
๎ = ๎๎
where ๎โฒ is the mass of the cart and ๎ is the hanging mass. To take friction into account, we add
to the right hand side the force of friction, ๎
๎
= โ ๎
๎
๎ = โ ๎
๎
๎โฒ๎. Then we get the modified
equation of motion:
๎๎ + ๎โฒ๎
๎ = ๎๎ โ ๎
๎
๎
๎
๎
๎
Solving for the coefficient of friction, we find
๎
๎
= ๎๎ โ ๎๎ + ๎
๎
๎
๎
๎โฒ๎
This provides us with a method for calculating ๎
๎
, the coefficient of kinetic friction between the
sliding surfaces.
We will use an inclined air track, two photogates, a counter/timer, a meter stick, a micrometer
and a balance and a collection of masses. The results you obtain from this experiment are highly
dependent on the accuracy of your measurements, so be careful!
Name:
Lab Partners:
Procedure
1. Level the air track. Do this by placing a cart on the track in the middle, turning the air
supply on, and adjusting the feet (evenly on each side of the track) so that the cart does
not move toward either end.
