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Name: Stephanie
Status: other
Grade: other
Location: OH
Country: USA
Date: Fall 2011


Question:
If two identical golf balls are dropped from the same height what difference (if any) would there be if one of the golf balls was spinning? (The reason why I ask this is because my daughter is a cheerleader who is tossed in the air and caught by a group of girls. They are working on a new skill where they drop her from approximately two people high. Right now, they just drop her and she seems to come down with a greater force, than when she spins as she comes down.)

Replies:
Hi Stephanie,

A spinning golf ball will tend to produce a force at right angle to the direction of the oncoming air stream. This is called the Magnus effect, and is caused by the interaction of the boundary layer around the spinning golf ball with the oncoming air. I do not think that this effect is at work with the cheerleader (rotation too slow, no appreciable boundary layer, non-uniform shape).

Another effect is aerodynamic drag. An object of non-spherical geometry which tumbles will tend to create more drag than if it were falling with no tumbling motion. So it might be tempting to think that because the cheerleader is tumbling then she is falling slower and catchers do not feel as much force when catching her. On the other hand, a drop of 10 feet (2 persons height or so) is probably not high enough to allow sufficient time so that a detectable difference in velocity builds up. Another factor to consider is that when the cheerleader is tumbling she is most likely holding her body in a very straight position, while when not tumbling she may allow her body to bend. This would also affect her drag coefficient (higher vs. lower, respectively), but again the question is whether there really is sufficient time for this effect to become large enough to be noticed by the catchers.

There may be other factors as well, such as the perceptions of the catchers who catch a tumbling objects as opposed to the non-tumbling object. Are they really sensing a difference force, or is the tumbling action just given them a false perception? If her body is not as rigid upon catching in the non-tumbling case, some catchers may feel more force while others might feel less. In general, the more her body tends to bend at the waist, the harder time the girls will have catching her since she will tend to slip through their grasp more. That means they will have to exert more force overall to catch her without slippage. Finally, are you sure she is being dropped from the same height each time? It would probably take more energy to loft a cheerleader with a tumbling motion than for a non-tumbling cheerleader (since she would have both a translational kinetic energy and an angular kinetic energy upon release), which may mean that she is not be lofted as high as a non-tumbling cheerleader. The last two possibilities (body position upon catch and maximum height) would probably have the greatest effect of all the possibilities mentioned.

Good question! Regards,

John C. Strong


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