Physics Lens

Concept Cartoons

Concept cartoons useful for eliciting rich questioning and classroom dialogue in a non-threatening manner. Speech bubbles can either represent possible problems and questions arising from daily interactions with science or surface alternative concepts of scientific ideas.

Here’s my attempt at doing one! It’s on the Physics topic of forces. There is both truth and error in what each student says. Can you identify them?

If you agree with Michael, how do you reconcile the reason given with the observation that in vacuum, both objects fall to the bottom in the same time despite having different weights?

Stuart Naylor from Milgate House Publishing and Consultancy Ltd will be coming to Singapore from 11 to 12 June 2009 to conduct a Concept Cartoon Workshop for Sec Science Teachers.

Suggested solution

While it is true that in vacuum, both balls will reach the bottom at the same time, in a fluid, there are other forces such as viscous forces (drag) and upthrust. Viscous forces would increase with velocity until the object reaches terminal velocity, when the sum of the upward forces of drag and upthrust and the downward force of weight are equal in magnitude. With the weight of the lead ball being bigger, it will take a longer while for the sum of the upward forces to equal its weight, assuming that both balls have the same acceleration in the beginning. Hence, the terminal velocity of the lead ball is larger than that of the aluminum ball and it will fall faster.

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