Day 89: Working on Work

Physics 11 – Working on Work was the topic for the day.  We had discussed in class how working is the idea of transferring energy into a system. As well, I had introduced the idea that energy is the area under a Force-displacement graph.  From this, it was relatively easy for the students to grasp that W=Fd.

The formula itself is easy to grapple with.  However, the nuances need clarification.  So went through 4 examples to illustrate 4 main points:

1. Work is calculated from applied forces, not a net force
2. Something has to move in order for there to be work. I briefly explained that even a transfer of heat is a type of work, where the “thing” that moves are all of the particles (as per the Kinetic Molecular Theory).
3. We only need to consider the component of the force that is in the same direction as the movement
4. Work, while being a scalar, can be represented as a negative quantity which refers to a transfer of energy out of a system.

After going through the examples, the class then did some Peer Instruction with Plicker cards.  One class went from 50% correct to 98% correct. The second class improved but still need an intervention – it went from 40% correct to about 60% correct.

I won’t use this question again, though.  There is in fact a small bit of horizontal force acting on the mass equal to the force of friction between the mass and the hand.  If μ = 0.1, then Ffr = 0.2 N, and the work done would be 0.4 J.  Only a few students asked about this, so I guess for one more year I avoided confusing too many people.