Jerking around
A car travelling along a straight road has constant acceleration from rest followed by a period of constant velocity finally braking to a stop. Sketch the displacement time graph. I gave this to my students today and before I collected the answers in I knew that most would be like the one in the picture (maybe not so colourful though). The problem is that students start to draw the first accelerating section but get it too steep and have to put a kink in the graph to get the straight bit on. The resulting motion would be,constant acceleration followed by a violent jerk, constant velocity, another violent jerk and eventually rest. The longer I teach the more I know what mistakes the students will make before they make them. Another common one is that students think that a moderator slows down the reaction rather than the neutrons. I was careful to point this out to my class but I must have confused them because most remembered it precisely the way it wasn't supposed to be. Maybe I should stop pointing out the pitfalls.
Tomorrow is peace day and I have to introduce some peace into my physics class. The topic I am supposed to be covering with my first year classes is Forces not a very peaceful topic really.