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Q & A: Why does my mom beat me in a downhill bicycle race?

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Most recent answer: 09/20/2014
Hello! I have heard that two objects with different mass will always hit the ground simultaneously if you dropp them from the same height at the same time. Because the extra force needed to move the heavier object is equal to the force gained (Galileo's Leaning Tower of Pisa experiment). But yesterday I was cycling with my mom. My mom weight twice as much as me. I cycled faster then her on flat areas and in uphills. But in downhills she was faster than me. My question is: Is it possible that she was she faster in downhills because of her weight? And if so: what is the difference between my mom and Galileo’s experiment?
- Anton Picetti (age 15)
Stockholm, Sweden

Hello Anton,

It's very likely due to air resistance.   Galileo's experiment was performed using very dense masses and the air resistance was small so the outcome comes out about right.   If you drop a feather and a lead ball in air the ball will fall much faster than the feather due to its density.   You can search u-tube and see demonstrations where the experiment is performed in a vacuum.  Sure enough, they both fall at the same rate.

Your mom is probably bigger than you.   Now the weight of a person is more or less proportional to the size cubed, whereas the surface area is only proportional to the size squared.   Since the air resistance is proportional to the area and the gravitational acceleration is proportional to the weight eventually weight wins out.  On level ground the speeds are much less that those of downhill racing and air resistance is not as significant.



(published on 09/20/2014)

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