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Dropping versus Spinning and Dropping
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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