Student Conception: Defying gravity

[Matt Anthes-Washburn]

Students may believe an object traveling fast enough can “overcome” or “defy” gravity.

Identifiers
Students will likely predict that the coin that is not moving as fast off the table will hit the ground first. Some will say that it will do this because it’s moving slower. Others may note that the faster-moving coin will have to travel a greater distance to reach the ground.

Confounding Experience
Sometimes we fail to perceive that fast-moving projectiles fall. Two examples are when a bullet is shot at a target or a pitcher throws a fastball. We think of those projectiles as going straight horizontally, and not following a parabolic trajectory.
A pop-culture example causing this misconception is the depictions in many cartoons.  Think of the Road-Runner and Wile-E Coyote running off of a cliff.  The coyote only falls after he has stopped moving horizontally. 

Formative Assessment Question
Separate two desks or tables by a few feet.  Challenge students to see who can slide a penny across the surface of one table fast enough to land on the surface of the other table.  Have other students verify that the coin does not get any lift in the sliding process. 
Ask any student who does not want to attempt the challenge to write down their reasoning.
Ask students to describe why this was a challenge.

Interventions
The activity provides concrete evidence that horizontal speed does not affect how fast an object falls to the earth. Students launch coins horizontally from a table, glancing one with the other, so they have very different launch speeds. In order to see this as a discrepant event, students must assess the strength of their experimental evidence. It is important for teachers to know that students tend to perceive the results they predict, even when presented with contradictory evidence.  One strategy to convince students that their results contradict their prediction is to have students predict not only which coin will land first, but by how many seconds the two landings will differ. To make the difference between the predictions and the results more dramatic, students can also try increasing the distance to the floor, with books or even off a balcony, if available. If it seems that one coin sometimes lands a little before or a little after the other penny, and they sometimes appear to land at the same instant, students should discuss what that reveals about the experiment.
(Gunstone, R., White, R., (1981) Understanding of Gravity, Science Education, 65, 291-299.)

[Lee Meadows]

Thanks for this, Matt. I appreciate how well you described this problem, and I hit it the same misconception with my students. It took a lot of time for them to work through it, but I think the time was worth the investment. Otherwise, they would have held on to this misconception for life.