Why is it that whenever a coin, like a quarter, is rotated that it always begins to circle around a point some distance from itself; it begins to almost revolve around nothing?
$\begingroup$
$\endgroup$
3
-
$\begingroup$ Do you mean, when you start spinning it on one spot, when it slows down, it starts moving from that spot in a circle? $\endgroup$– user81619Commented Aug 6, 2015 at 21:57
-
$\begingroup$ @AcidJazz that's how I read it. $\endgroup$– Kyle OmanCommented Aug 6, 2015 at 22:33
-
$\begingroup$ Useful - en.wikipedia.org/wiki/Euler%27s_Disk $\endgroup$– QCD_IS_GOODCommented Aug 7, 2015 at 0:15
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
4
Based on my guess above, this question may answered by the following steps, based on the transfer of spin angular momentum to orbital angular momentum and also the conservation of total angular momentum:
Firstly, you provide angular momentum to the coin by twisting it with your fingers and it has enough angular momentum to remain upright.
It basically stays in that one spot, revolving around the spin axis until it start to lose it's angular momentum through friction with the air and the ground.
As it slows down, the spin axis will start to precess,i.e. to move in a circle, swinging further and further away from the original spin axis until it's bottom pivot point starts to shift on the ground and move away from the original pivot point.
The coin has started with almostly totally spin based angular momentum, but this will be transformed, as it topples over, into orbital angular momentum, which will cause it to roll around the floor.
The coin will then rotate in a spiral shape, possibly starting off in almost completely circular "orbit", but it will move inwards to eventually, as you say, revolve around nothing.
It is the same efffect as a gyroscope undergoes, with the difference that a gyroscrope has a point pivot, so instead of actually moving in a circle, it drops to the floor, the gyro frame spins around a few times, and stops.
Because the coin can undergo rolling motion, it travels around the floor in a circle/spiral instead.
One idea you could try, because I can't at the moment find any web based footage, is to film the coin and then, if you can run the clip slowly, you may get a better idea of the process in action. YouTube should have something like this, somewhere.
Found It:
-
$\begingroup$ Why is it that when I spin a coin slightly off axis quickly enough the axis of rotation will straighten itself momentarily, before beginning to drop as it loses angular momentum? $\endgroup$ Commented Aug 7, 2015 at 0:19
-
$\begingroup$ Maybe its the mass distribtion of the coin, i.e., you get it just right sometimes, so that an uneven mass distribution "sorts things out", even momentarily. I am trying to think of bigger objects than coins that this effect occurs in, but think of the motifs, wording etc stamped on the coin, they must affect the balance so that it depends on which axis you happen to spin it along. $\endgroup$– user81619Commented Aug 7, 2015 at 0:25
-
$\begingroup$ This effect happens in all coins from various nationalities I have, plus the video that you linked with the guy flipping the coin onto the table, the chances that he gets the axis perpendicular to the table is pretty much 0, except the coin always manages to right itself if he flicks it quickly enough, so I think it's an intrinsic property of spinning coins. $\endgroup$ Commented Aug 7, 2015 at 0:28
-
$\begingroup$ Then it's inertia in some form, I would guess, I would love it see it for more massive, slower objects, a coin is just too small and fast for me to follow. If I have you right on when I spin a coin slightly off axis quickly enough the axis of rotation will straighten itself this to me anyway, implies precession is "straightened out".. $\endgroup$– user81619Commented Aug 7, 2015 at 0:38
$\begingroup$
$\endgroup$
What you are describing may be caused by gyroscopic precession which results from unbalanced torque applied to the coin when you begin its spin.
When you spin a coin, you transfer a moment of torque to the coin by putting pressure on its edges.
For example, if you hold the top of the coin between your thumb and forefinger, your thumb twists the upper edge away from you, and your forefinger twists the reverse side of the upper edge toward you. Clockwise motion results.
But it is highly unlikely that you will be able to exactly balance the pressure of both fingers to produce a moment of torque exactly about the vertical axis of the coin. As a result, the coin begins its spin with slightly more pressure on either the heads or the tails side. Thus begins a wobble which grows more pronounced as the spin slows down.
The wobble unbalances the coin about its axis and causes the axis to wander over the surface the coin is on.
Helicopter pilots need to be aware of the effect of gyroscopic precession, as a force applied to the edge of a rotor can cause the rotor to start wobbling and unbalance the aircraft. Here is a link to a diagram showing the result of pressure applied to one or the other edge of a helicopter rotor: http://www.copters.com/aero/gyro.html. As you can see, the upward or downward pressure (sideways in the case of a spinning coin) causes the rotor to respond by deflecting from the plane of rotation 1/4 turn later.
When his happens in a helicopter, the axis of the rotor starts to wander, same as the axis of a coin wandering over the surface on which it spins.
Acid Jazz's answer better explains the deteriorating rotation which becomes orbital and then wandering.
