by reyrey389
Tags: confused, inertia, moment
 P: 11 I=mr^2 I know moment of inertia depends on how for a object how far a bit of mass of that object is from the axis of rotation. i.e. farther from the axis = higher moment of inertia-= more torque is needed to cause rotation. this just doesnt make any sense for the seesaw example, on the seesaw the farther the object is from the axis of rotation, the LESS torque you actually need. (you need more torque closer to the pivot) this e.g. just contradicts my first sentence. can you please help i've been confused on this for quite awhile...
 P: 450 Torque is not something that is "needed to cause rotation". Torque is the rate of change of angular momentum and a good way to think about is, is like you do about forces. The second law is F=ma similarly T=lα where α is the angular acceleration. When you apply a force F, on two different masses , the acceleration of the smaller mass will be higher. In the same way if you have two object of different moments of inertia and you apply the same torque the angular acceleration of the smaller l will be higher.So in the case of the seesaw if you want to have the same angular acceleration for a mass close to the pivot and one far. You will need a higher torque for the far mass and a smaller torque for the close one.
 P: 1,877 Moment of inertia is telling you how your mass is distributed throughout your object (it literally is a moment), and, therefore, how angular momentum and energy act on it.
P: 84

A good way to think about this (given by Richard Feynman) is to think about a door, partly open. With one finger try to close it by pressing hard near the hinge edge - almost impossible. Now apply the same force near the edge away from the henge - easy.
HW Helper
P: 4,523
 Quote by reyrey389 for the seesaw example, on the seesaw the farther the object is from the axis of rotation, the LESS torque you actually need.
The farther your friend is from the pivot point, the MORE torque you need to balance his weight.