It's All Friction

Studying the motion of a cart as it rolls up then back down a ramp, students noticed what they called an "anomaly," that the slope of a velocity vs. time graph is different for the upward vs. downward motion. They split into groups to study this, and then came back together to discuss their findings.

Ellie suggests that surface friction matters more than air friction, based on the surface friction group seeing a bigger effect than the air resistance group. Kapp takes issue with that, arguing that surface friction doesn’t always have to be larger than air resistance. Emilia tries for consensus, “it’s all friction,” which draws laughter, and Marka more specifically offers that an increase in friction “in general” explains the difference in slopes.

Next, David P interjects with a question for the WD-40 group.

David P’s question shifts the focus to the WD-40 group’s experimental procedure. David R clarifies that he wiped down the ramp for the “control” before using WD-40; he had to concede they didn’t have a “real control.” Emilia wants to do another test, but Erica asserts they don’t need to, since all of the other tests have resulted in the “anomaly.”

Next, the teacher asks how the weight group’s results fit into the conclusion the class has drawn.

The Weight Group had done 4 trials with increasing weight. They expected adding weight would increase friction between the cart and ramp, in turn increasing the difference in slopes (in other words, enhance the “anomaly”) on the velocity-time graph.

Marka points out that the weight group did observe a trend for 3 of the 4 data points, so the first run may have had some experimental error.

Isaac suggests that maybe friction did change, but not enough to see a difference since the track/cart combination is designed to be low-friction. Sarah resists dismissing the discrepancy because if friction is even just increasing a little bit, they should still see a small increase in the anomaly for each trial.

Next, Kapp questions the weight group’s assumption about weight affecting friction.

Kapp has a very different idea, that weight could have the opposite effect: snow would “hold back” a light car more than a heavy car. Kapp’s example inspires comparisons to other situations: Marka’s cardboard box example supports Kapp’s idea, but David P’s example of a Prius goes back to weight increasing friction..

Emilia thinks of a bowling ball on a pillow, how it would make more of a dent than a light ball. Max turns Emilia’s pillow surface into a “pillow ramp,” to picture a ball rolling, which would be easier for a beach ball than a bowling ball.

Isaac’s “rotund” and “skinny” skaters example supports what he said earlier – a small difference in friction wouldn’t matter. Sarah brings up inertia again, suggesting that inertia may be changing with weight , not friction, which would affect the cart’s motion somehow. Kapp adds on, saying maybe we are changing both friction and inertia, and we don’t know if one affects the motion more, or if they “balance.”