The force conversation of angular momentum creates

Main Question or Discussion Point

A body is tied to a wire that go through a hole in a (frictionless) table on which the body is spinning.

The body is spinning while the wire is pulled from beneath the table.
according to conversation of angular momentum the speed of the body should now increase, but in terms of forces... what force is actually increasing the speed? the force of the wire can't do it since it perpendicular to the velocity, (thus not doing any work, and thus the kinetic energy should be the same).

so what force is doing it, and where does it comes from in the atomic level..?

Thank you

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Dale
Mentor
the force of the wire can't do it since it perpendicular to the velocity
If the object is being pulled in then it is not moving in uniform circular motion and the force of the wire is, in fact, not perpendicular to the velocity and therefore does work on the object.

If the object is being pulled in then it is not moving in uniform circular motion and the force of the wire is, in fact, not perpendicular to the velocity and therefore does work on the object.
damn, I actually thought about it, and some why deny it..
because seemingly in the first infinitesimal moment it still perpendicular.

so what about gyroscope, that stand still in unnatural way
http://t2.gstatic.com/images?q=tbn:...aviddarling.info/images/gyroscope_diagram.jpg

you have mg pulling it down, why doesn't it fall?

Dale
Mentor
you have mg pulling it down, why doesn't it fall?
It does not fall down because at the pivot there is also a normal force equal to mg pushing it up. So there is no net force in the vertical direction.

It does not fall down because at the pivot there is also a normal force equal to mg pushing it up. So there is no net force in the vertical direction.
but.. what?
the net torque is not zero..
and what the different between a spinning gyro and a non spinning..?

Dale
Mentor
Force and torque are different. The net force is 0, the net torque is not zero.

The torque is the cross product of the weight (down) and the vector from the pivot to the center of gravity. This vector is in the horizontal direction, and therefore the angular momentum changes in that direction which causes the gyroscope to precess.