Over the weekend I glued some sandpaper to some wooden blocks I happened to have in the garage and screwed some eye hooks into them to make friction blocks for today’s lab.  The students were to find the coefficient of static friction for two different block/surface combinations; sandpaper block on a bare wood surface and wood block dragged on a wood surface.  As you can see in the photo they used spring scales to measure the maximum force they could exert on the block before it started to move.   In the diagram below you can see the block sitting on a table and the 4 forces acting on the block.  The force of gravity, or weight of the block and the normal force must cancel each other out since the block is not moving in the vertical direction.  The normal force is just the force exerted by the table on the block.  If the block is not moving, then the two forces along the horizontal, the force we’re pulling on the block and the force of friction which is resisting that pull, must  cancel as well.  Eventually we will pull hard enough that the block starts to move. The students measured the maximum force which they could exert on the block before it moved.  They also measured the mass of the block with a triple beam balance so they could calculate the force of gravity and hence the normal force.  Once they had the normal force and the maximum pulling force they solved for the coefficient of static friction, μs.

Have you ever had to move a very heavy object? When you push on it a little it doesn’t move, but when you push really hard and finally get it moving you find you don’t need to push as much to keep it moving.  When an object is moving the frictional force is smaller because the coefficient of kinetic is smaller than the coefficient of static friction.  The students did the experiment again but this time they pulled on the block to keep it moving at constant velocity.  If the velocity is constant, then the net or total force on the block is still zero and they could use the same formulas as before to find the coefficient of kinetic friction.

Reading spring scales while pulling on blocks isn’t the easiest thing to do so the students also used a pulley system to exert the force on the block. This was a more accurate method, but it took a bit longer.  The results compared pretty well with the spring scale measurements.