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Name: Carina
Status: student
Grade: 4-5
Location: MN
Country: USA
Date: April 4, 2011


Question:
On a marble race track, what marble will go faster, a big marble or a small marble and why? The big marble is a little bit heavier. In my experiment, the small marble went faster in 2 tracks out of 3



Replies:
Carina

The momentum of a body is its mass times its velocity. (p = m x v) So if you apply the same impulse of momentum to each body then the body with the smallest mass will have the highest velocity. Thus the smaller marble will go faster if you apply the same impulse of momentum to each body.

Sincere regards, Mike Stewart


I am not sure there is one prefect answer, Carina.

A big marble is better at pushing through the air. So it might be faster. Most scientists would guess that the big marble should be faster. But a big marble might make your race-track sag and flex more, if your track is plastic or made of straws. That would loose some energy, and make the big ball slower.

If the big marble is rougher than the little one, that too might make the big marble slower. Each little bump in the roughness on the marble makes the marble bounce up and down a little, and then the bouncing dies out like any bouncing ball, and that uses up energy.

I think there might be another reason too, that I did not think of yet. I hope you try thinking about what the big marble does differently than the little one. Maybe you can see or hear some good clues as each ball rolls down the track.

Jim Swenson


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