Day 72: I’m sensing some friction and ‘Tom Petty’ visits

Well, a Monday morning… what better way to start than with a force diagram assessment.  Here is the one we did today:

The majority of the students did pretty well, but there are still a few that need to come in for help, but just will not… like the two that had a frictional force but NOT a normal force !

After the assessment, we had a pre-lab discussion related to the frictional force.  We decided that we could quantify the (kinetic) frictional force if we pulled a block with a string with a force sensor.  Two conditions had to be met.  First, the string had to be parallel to the surface so there were no components to the tension force. Second, it had to be pulled such that the block had a constant velocity.

We discussed the factors that might affect the value of the frictional force… the usual suspects showed up; mass, weight, surface area, velocity, types of surfaces.  We discussed that we could not change the mass without changing two other variables and that is a major no-no in a controlled experiment.  Changing the mass will also change the weight and the normal force exerted on the block by the surface.  After some guiding questions, we decided to look at the normal force exerted by the surface because we can change that without changing the weight and mass.  We just need to add an extra normal force that points down on top of the block… we could just push on it, but that is hard to keep constant and measure.  A student (thankfully) suggested putting a known mass on top of the block, but still continue to define the system as the block.

To save some time, I demonstrated how to gather the data and showed that for us, there is no effect with the velocity or the surface area.  Tomorrow the students will vary the normal force on a given set of surfaces.

General Physics:

I collected the one problem assessment I sent them home with last Friday– solve a constant acceleration problem with the kinematic equations. I messed up a bit because the one I made up (based on a goal-less problem) was too hard for most of them.  Now I will need to give them a another to show they really can do it.

We then looked at an example of constant acceleration, namely freefall. I bridge this from our ramp lab.  We sketch the stack-o-graphs for increasingly steeper ramps until we have a vertical ramp.  To gather the data, we use the picket fence and photogate. I do not worry about if they fully understand how the photogate gives us the v-t graph, just that it is the slope of it that we want.  As part of this, we add washers to the bottom of the picket fence to increase the mass to see if there is any effect.  Tomorrow we share results.