# Spin and Rebound Heights

1. Oct 23, 2006

### GreenSabbath

My Experiment consists of two balls of different masses falling together in the y axis from the same fixed height.Im researching how mass of the smaller ball affects its rebound height. My method of adding mass however was very crude as I stuck weights on using tape. This surely altered the aerodynamics of the ball and I did notice an increased spin on the ball, (unbalanced weight arrangeent?). What does this mean? The energy given by the larger ball is used up in giving spin and velocity to the samleer ball hence the rebound height is lower than what it hypothetically should have been?
My readings are very far off the elastic model i formulated, barely even following the trendline. How do i explain this?

2. Oct 23, 2006

### moo

I think your observations are correct.

Unbalanced balls have an extremely "wonky" bounce. The lopsided impact causes rotation which absorbs much of the energy normally used in the rebound, plus the spin gives it a partially horizontal bounce rather than all vertical (which trades some height for distance).

If you want anything resembling accurate results, I'd say get more balls...

moo
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Last edited: Oct 23, 2006
3. Oct 23, 2006

### GreenSabbath

What causes this "wonky" bounce, alteration in a ball's aerodynamics?
How can I explain it assuming lets say I add 200 grams on the two opposite ends of a ball (x axis) but they arnt exactly aligned ... will the ball rotate around the y axis? Adding weights causes the spin because area of the body is proportional to air drift?
And why did the spin increase as heavier weights were placed onto the ball?
Does adding weights change the moment of inertia which ends up altering spin?

Last edited: Oct 23, 2006
4. Oct 23, 2006

### moo

When the ball impacts, the heavy side doesn't "stop" when the lighter side does, so the ball pivots a bit, and this continued movement by the heavier side translates into rotation.

If the ball is dropped from a good height then surface aerodynamics will also come into play, but I doubt it's a factor from a few feet unless you really have a large surface difference.

moo
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moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

5. Oct 23, 2006

### GreenSabbath

Thanks moo .. really appreciated

6. Oct 23, 2006

### moo

You're welcome.

Btw, "wonky" is not a technical term...

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

7. Oct 23, 2006

### GreenSabbath

Would the center of mass change?
What about moment of inertia ?
How can I link the increase in the added mass to the spin of the ball?

For a full sphere, I = 2/5 m r2
By adding two bulk wieight on either side am i changing the value for alpha?

Last edited: Oct 23, 2006