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Question about precession in the case of frisbees

by Thundagere
Tags: case, frisbees, precession
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Thundagere
#1
May9-13, 10:53 PM
P: 159
So, I was trying to figure out the physics of frisbees. When you throw a frisbee at the ground at an angle (such that the side to the left of you, assuming you are right handed, hits the ground first), then it will suddenly skip up and fly upwards (Youtube has some great videos of it, for anyone who hasn't seen this). Someone told me this was because of precession. How does this work?
Precession is the phenomenon where the angular momentum changes in the direction of a net torque, correct? In this case, we have an upward normal force, so a torque horizontal to the ground going away from the thrower on the edge of the frisbee. The angular momentum vector is, of course, through the axis of rotation. They're almost 90 degrees to each other, so how would precession work in this case to flip the frisbee up?
Thanks for any help!
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rcgldr
#2
May9-13, 11:36 PM
HW Helper
P: 7,109
The frisbee is just bouncing off the ground, the same as any semi-elastic colliision. Precession also occurs, but almost all of the change in direction from the ground is due to bouncing.
256bits
#3
May10-13, 07:34 PM
P: 1,472
In this case, we have an upward normal force, so a torque horizontal to the ground going away from the thrower on the edge of the frisbee. The angular momentum vector is, of course, through the axis of rotation. They're almost 90 degrees to each other, so how would precession work in this case to flip the frisbee up?
Exactly.
From above the frisbee is rotating clockwise, so the rotation vector of the frisbee points down.

The normal attempts to twist the axis of rotation of the frisbee clockwise from your perspective.
So you have a torque vector from this normal pointing away from you.

From that you can deduce the direction of the resulting precession vector which will cause the frisbee axis of rotation to change direction and cause it to what looks like a skip and fly upwards.

Here is a site that explains how the three vectors are related:
http://www.motivate.maths.org/conten...oscopic-effect

Bobbywhy
#4
May10-13, 09:23 PM
PF Gold
P: 1,894
Question about precession in the case of frisbees

256bits, thanks for that website reference! One of the better explanations I've ever seen!
rcgldr
#5
May11-13, 12:32 AM
HW Helper
P: 7,109
Quote Quote by 256bits View Post
From above the frisbee is rotating clockwise, so the rotation vector of the frisbee points down. The normal attempts to twist the axis of rotation of the frisbee clockwise from your perspective. So you have a torque vector from this normal pointing away from you.
So the result is a pitch upwards of the frisbee, causing it to climb, but there is an initial bounce, and I wasn't sure if you were asking about the bounce or the tendency to climb afterwards.

Also, for a cambered airfoil like a frisbee, there's a pitch down torque. This is somewhat compensated for by having the center of lift in front of the cetner of mass, but generally it's a pitch down torque, that results in a roll reaction (precession) during longer flights.

A flying ring, such as an aerobie, solves this problem and can be "tuned" so that it does not roll during long flights.

http://aerobie.com/about/ringscientificpaper.htm


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