Basic question

tucky

Hi everyone this is hopefully a quick and easy question:

If a person spins an object that rotates, is that considered torque?

When that object that is rotating, are the forces that act upon it: gravity, air resistance, friction, and the beginning twist from that person, newton’s second law?

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arcnets

If a force acts upon an object so that the object's rotation changes, then the torque is
T = dL/dt.
So, if the person speeds up the object's rotation then, yes, there is a nonzero torque.

Concerning Newton's laws, I never know which is which, so please help me.

tucky

Newton 2 law F = ma F = I* alpha

arcnets

I see. Should read
T = I[alpha].

Now you ask "are the forces ... Newton's second law".
Sorry for nit-picking a bit, but of course forces cannot 'be' a law.
Better to ask: "Does this law apply to a situation in which these forces act on a spinning object".

My answer is: "Yes, but."

Here's the 'but':
T and [alpha] are both vectors. So I must be a tensor (something representable by a matrix).
This means, physically, that a rotating body does not have >it's< moment of inertia I, but I is defined WRT an axis.

In general, a spinning top will not have a constant axis of rotation, but will show nutation and precession. Just think of a gyroscope.

OK, you could say "I do not care about nutation and stuff, I just assume the axis stays the same all the time".

In this case, |T| = I|[alpha]| (a scalar equation) is correct.

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