In physics class today, our teacher wanted to demonstrate that mass doesn't have an effect on fall speed. (The class was about gravitational potential energy becoming kinetic, mgh = 1/2 *mv^2 and all that)(adsbygoogle = window.adsbygoogle || []).push({});

Here was the set up: He had a small rail (two parallel thin metal poles) sloping down, becoming completely horizontal at the end, then leading off of a table. He would roll a dense metal spherical ball bearing down the rail, it would roll down the slope, roll off with a horizontal velocity and roll off of the table onto the floor. Where the ball would land, he had a piece of carbon paper over a piece of copy paper, such that the ball would leave an imprint right where it landed.

He rolled a small ball bearing and a larger metal ball (3 or 4 times the radius). But each time he tried, the metal ball would always travel 4 or 5 inches further than the small ball. He tried lining up the middles and being careful in general, but the large ball always travelled farther. The teacher was a little bit disturbed.

Air resistance and friction should be negligible in this example.

So... Why should the large ball go significantly farther?

I thought about this for a bit. I thought that maybe some of the energy in the larger ball was going into rotational kinetic energy. But as the velocity of the center of mass is equal to the tangential velocity of rotation, the kinetic energy lost to rotational kinetic energy should be proportional for both the large and small balls. In other words, I don't see how the radius would affect the velocity.

So what gives!?

Thanks.

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Radius of ball matters in fall speed!

**Physics Forums | Science Articles, Homework Help, Discussion**