College-Prep Physics: I didn’t anticipate spending a whole period about how a fan cart works. Nor was I expecting students to say a fan cart’s speed would accelerate up to a constant speed.
Transfer is hard. We had already done the bowling ball and mallet activity. But this wasn’t a bowling ball. It was a fan cart. In their minds, different objects means different outcomes.
That’s when I came up with the idea of “physics goggles.” When you’re wearing your physics goggles, you see past the superficial features of different situations and focus on the underlying principles. In the case of the fan cart, the air molecules were providing the “taps” to accelerate the cart. And since those “taps” never stop, the cart would continually increase speed, just like a tapped bowling ball.
While students agreed that you could get the bowling ball to go as fast as a Ferrari (even with tiny taps, as long as you could keep up with the ball), they initially struggled with the fan cart going as fast as a Ferrari. Most thought the top speed of the fan cart was determined by the speed of the fan.
“Time to put on our physics goggles!”
AP Physics C: Here’s the velocity-time graph for a fan cart that received a brief push from my hand, then slows down as it rolls away from me, picks up speed as it rolls back to me, then is stopped by my hand. Notice that the acceleration while the cart was rolling away from me is NOT the same as the acceleration when it rolled back. This is because friction is not negligible and changes direction. A free body diagram for the away and return trips will show you how.
It was then put as a challenge to students to use the momentum principle and the data from the Logger Pro file to find the magnitudes of all the forces acting on the cart:
- the gravitational force from the earth
- the normal force from the track
- the thrust from the air
- the frictional force from the track
Later, we’ll create a model in VPython and check our model data against our laboratory data.