Students were told to read Section 2.2: Wheels in How Things Work: The Physics of Everyday Life or Chapter 7: More About Forces in the Cartoon Guide to Physics. I requested the students watch the first 5 minutes of Crash Course Physics on Friction.

And some videos by Doc Schuster:

We pretty much jumped right into the lab activity today, which was to measure the coefficients of static friction, μ_{s}, and kinetic friction, μ_{k}, for a block on a surface (table cloth, cardboard, rough wood, whatever). There were a couple of ways students could make the measurements. The first involved placing a block on the surface and tying a string from the block to a weight hanging over a pulley. The amount of weight hanging off the string is the pulling force on the block. If the block is not moving (a = 0, therefore net F = 0) then the force of friction must be equal and opposite to the pulling force. Likewise if the block is not accelerating up or down then the normal force is equal and opposite to the force of gravity, the block’s weight. The weight of the block is measured in Newtons on a spring scale. If we had been doing the experiment on a ramp, then the normal force would NOT be equal to the force of gravity, we’d have to take into account the angle of the ramp – but today we just dragged the blocks horizontally. To measure the coefficent of static friction the students had to find the maximum pulling force they could exert on the block WITHOUT making it move. Once the block starts moving the force of friction is now kinetic friction, and is less. We’re trying to find the maximum force of static friction, F_{static} = μ_{s}F_{Normal}, how much force do we have to overcome to make the block move. When they had that force, roughly 1N, they divided it by the normal force of the block to find the coefficient of static friction. The coefficient of frictions, static and kinetic, depend on the two surfaces. If the block is rough like sandpaper and is being dragged over a rough wood board, then the friction is going to be higher than if the block has been sanded smooth and is being dragged on a polished surface.

The coefficient of kinetic friction should be less than static friction, its easier to keep an object moving then to get it moving in the first place. Think about trying to move something heavy like a fridge, at first it won’t budget until you exert a very large force, but once you have it moving its easier and you don’t have to push so hard. To measure the force of kinetic friction, students dragged the block on the same surface at constant speed with a spring scale. If the block is at constant speed (at least roughly constant), then its acceleration and net force must be zero, so the force measured on the spring scale is equal (opposite direction) to the force of friction on the block.

Just as before they can now find the coefficient of kinetic friction, F_{kinetic} = μ_{k}F_{Normal} by dividing the force on the spring scale by the normal force (weight they measured earlier) and indeed it was less than the coefficient of static friction.

The third measurement involved using the Go Direct Force Sensor from Vernier. Its basically a digital spring scale that sends the data directly to your iPad via bluetooth. Using this sensor students collected data while they slowly increased the force on the block until it started moving and then tried to keep it moving at a constant speed.

The graph of the data looks like this:

As the student increases the force on the block the Go Direct sensor records the increasing force, indicated above with the red line, but the block has NOT started moving yet. As the force reaches 0.72 N, the block starts moving and the force drops to 0.52N (green line). From this one data set students can get read off the forces required for both the static and kinetic coefficients of friction.