# Elementary science project - HELP!

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1. Feb 26, 2010

### KPalmer

I am looking for an easy way to explain to my 2nd grade son which ball will roll down a ramp faster. I have a ramp (more like a trough) set at about 30 deg angle incline. He is going to roll a baseball, pool ball, golf ball, and basket ball down the rramp. He is only 2nd grade so I can't get into the math since he hasn't had multiplication yet. Any suggestions? - This is his science fair project.

2. Feb 26, 2010

### Bob S

This might be too advanced for a second grader. The acceleration down the ramp is gÂ·sin(incline angle) minus retardation due to the moment of inertia. Solid spheres have a moment of inertia 2mR2/5, while hollow spheres have moment of inertia 2mR2/3. See

http://en.wikipedia.org/wiki/List_of_moments_of_inertia

The hollow basketball will (should) roll slowest.

Bob S

3. Feb 27, 2010

### Naty1

Pretty complex for a second grader....

Maybe you can contrast the difference relative to free fall???...free fall acceleration due to just gravititational inertia (mass)...versus and the combined rotational inertia and gravitational acceleration [g sin (angle)].....

Think about your trough in a vertical position (free fall objects) and then, the other extreme, horizontal, to contrast the different behavior...and an angle in between....

equal acceleration in the first free fall, no acceleration in horizontal, different acceleration at the "angle". You can illustrate in an angle fall some objects fall slower than others....the slowing is what's called Bob's rotational interia.

I can't think of a simple way of contrasting rotational inertia with free fall inertia....maybe check on gyroscopes??