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:
- Work is calculated from applied forces, not a net force
- 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).
- We only need to consider the component of the force that is in the same direction as the movement
- 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.