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# Moments

Introduction to moments. Created by Sal Khan.

## Want to join the conversation?

• why is F capitalized and D is not?
• Its just the symbols for force it is 'F' and for displacement it is 's' or 'd'
• what is the difference between moment and torque? are the both same ?
• Moment refers to inertia, ie. rotational mass. Depending on the axis of rotation, you have different moments for the same mass. Thus inertia is a tensor quantity.
• At , he said counter-clockwise, drawing an arrow depicting clockwise. Shouldn't it point the other way?
• It's also ok because if he consistently gets all of them backwards, he'll get the right answer again. :)
• Can somebody tell me what exactly is Axis of Rotation?
• It is an imaginary line around which the object is rotating. For instance in a door, the axis of rotation is the line that passes through the hinges. When we speak about the rotation of the Earth, is the line that passes through the Poles.
• ok, this is kind of a stupid question, but he said that F1d1=F2d2. But that's only when it is in equilibrium, right?
• That's really the same thing as saying that they're still in equilibrium with all three forces. If the system isn't in equilibrium, you can solve for the net torque if you know the angular acceleration it causes as well as the rotational inertia of the object (T=Ia (not the same "a")). Think of this as the direct equivalent of typical translational forces, where F=ma.
• how to calculate the reaction force if you have many forces acting on one object
• You have to use vector analysis to figure out what the net force is.
• A SPHERE cannot roll on a SMOOTH INCLINED SURFACE . Why ?

Thank you .
• What is a Pseudo-vector?It is mentioned by Sal Sir at .
• A vector-like object which is invariant under inversion is called a pseudovector, also called an axial vector (as a result of such vectors frequently arising as vectors describing rotation. Hope This Works!