# Day 74: Frictional Force Post Lab and an AWESOME Free fall movie

We had the post lab discussion for the frictional force experiment. This is not a tough discussion for the students because of the way the experiment was initially set-up.  Essentially, we varied the normal force and measured the frictional force for a given set of surfaces.  The graph is linear and it is different for the two sets of surfaces we tested… conclusion= the types of surfaces do make a difference.  This is something most of the students fully realized, but now the  slope of the linear graph provides a way for us to quantify the ‘surface effect’.  That’s how we conceptually defined the slope, then gave it the proper name, the coefficient of kinetic friction.

Another important aspect of this experiment is figuring out why there are two types of friction, kinetic and static, (and why they are not the same value). We have been drawing force diagrams for these for close to two weeks, now we are in a position to explain it.  I simply complete one trial of the experiment the students had just completed (making sure there is a large mass on the block). The students watch the data gathered in real-time on the screen and I ask them to determine when the block finally starts in motion.  Here is what the resulting graph look like (with my added scribbles): It is easy for the students to see that the block does not start in motion until just after the sharp peak. So this leads us to a second coefficient of friction for each set of surfaces, the static coefficient.   A number of student groups commented that they saw the peak also and wondered about it.

General Physics:

We started the period by watching this short movie clip, which is totally awesome:

I had attempted to demonstrate this with a piece of paper and a textbook, but this is much better.

Following this we, WB’ed a set of free fall problems.