center of gravity?

[mcoy]

i got a question, if i spin rapidly a 2kg book on my fingers, why does it seem to be lighter than when its stationary??

[Ophiolite]

Because the incoming sensory data is different when the book and your fingers are moving than when they are stationary.

[5614]

from the sounds of it only the book is moving and is finger is still.... how would the book moving effect sensory data in that way?

 

unless its got some kinda lift effect, such as with a helicopter.

[Ophiolite]

Hold your hand up, fingers spread, then place a book on top of them. Now spin the book with your other hand. If you are telling me that feels the same then you are not paying close enough attention.

[5614]

well you can feel the book spinning around on top of your finger.

 

as i said:

"how would the book moving effect sensory data in that way?"

 

the "in that way" part was because obviously it feels different but how does a spinning sensation on the tip of your finger make it seem to weigh less?

 

(you dont need your fingers spread and you put your other hand back down beside you - presumably)

[Janus]

I would say this. While the book is not spinning you are constantly having to exert effort in keeping the book balanced, by slightly moving your fingers as the book tends to topple to one side or the other.

 

When the book is spinning there is a gyroscopic effect that tends to keep the book balanced for you. You don't have to do the work of keeping the book balanced, so the book seems "lighter".

[mcoy]

good point, maybe its just some upward thrust from the air (dunno what its really called)....coz when i spun a lighter and wider book it flew off....

[DavidAngelMX]

Don't you think this situation is pretty much like having a coin sipinning on its side? While we keep the book spinning, it has a rotational momentum L due to ratation horizontally, which we can say points up, and there is a gravitational torque T, tending to produce a rotational momentum L' pointing horizontally. By summing L and L'(t) we obtain a changing ?L that points diagonally, adjusting the object's rotation in a way that fits this new changing torque, just as an always orthogonal force deviates a particle instead of changing its speed.

 

This new rotation implies something like a lifting force, but I can't realy explain because I can't write any equation here, so I guess you'll have to solve this or look at it in a book. I recommend University Physics, Sears-Zemansky-Young, or its "Scientific-Tecnical Collecton" Mechanics, Heat and Sound, Sears. This is where I've seen it best explained. You can also try Physics, Vol.1, Tipler.

 

The thing is that it is (I suppose) the net rotational momentum ?L that makes the book easier to hold, it has nothing to do with air, though it might add a little lift too, but that depends on the shape of the book and the way you hold it.