Physics Day 3: Position and velocity graphs

Today when position graphs, one student asked how the graph would change if the origin was different. He thought the position graphs would no longer be linear. I had students talk in groups about what the graph would look like if the origin was between A and B’s starting positions. I had a student come draw her new graph on the board. Instead, she just drew shifted axes on the same graph – smart! image

(And after further probing, I learned that the student who had the question was actually thinking about 2-dimensional space, and what if the origin was not on path that A and B were traveling. good question!)

Later, students were concerned about making a velocity graph from a position graph that starts at time t=0. Some students really wanted to use closed circles or open circles for the endpoints of the lines. They thought if the position graph endpoint is a closed circle, than the velocity graph would have to start with an open circle, since you can’t actually find the slope of the position graph at t=0. Idea on how to respond to this other than saying there’s things math teachers care about that physicists don’t? Maybe that the slope immediately after t=0 is essentially the same as the slope at t=0?


Later in class, I was showing how you could solve the position equation for velocity, and you got back to the slope of the position graph. There was an audible gasp from multiple students!


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