- #1

- 30

- 0

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?

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter tucky
- Start date

- #1

- 30

- 0

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?

- #2

- 508

- 0

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.

- #3

- 30

- 0

Newton 2 law F = ma F = I* alpha

- #4

- 508

- 0

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.

Share:

- Replies
- 1

- Views
- 229