Is spin the same as torque in physics?

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Homework Help Overview

The discussion revolves around the concepts of torque and spin in physics, particularly in the context of rotational motion and the application of Newton's laws. The original poster questions whether the act of spinning an object is equivalent to applying torque and how various forces interact with the rotating object.

Discussion Character

  • Conceptual clarification, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the relationship between torque and the forces acting on a rotating object, including gravity, air resistance, and friction. There is an attempt to clarify the application of Newton's second law in this context, with some participants questioning the definitions and relationships between torque, angular acceleration, and moment of inertia.

Discussion Status

The discussion is ongoing, with participants providing insights into the mathematical relationships involved in rotational dynamics. Some guidance has been offered regarding the definitions of torque and angular acceleration, as well as the implications of vector quantities in rotational motion.

Contextual Notes

Participants are navigating the complexities of rotational motion, including the distinction between scalar and vector quantities, and the effects of various forces on a spinning object. There is an acknowledgment of the potential complications introduced by phenomena such as nutation and precession.

tucky
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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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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.
 
Newton 2 law F = ma F = I* alpha
 
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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