Question #07729

1 Answer
Jun 19, 2017

A moment is a reactionary torque which opposes rotation about a given point as generated by outside forces.


Consider Newton's first and third laws of motion:

  • An object at rest stays at rest until acted upon by an outside force
  • For every action, there is an equal and opposite reaction

If we thought about a block undergoing linear motion, a force acting on the block would cause the block to move only when the frictional force opposing the motion is exceeded by the applied force.
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This idea can also be applied to rotational motion when a torque is applied to a beam. If when a torque is applied to a beam, the beam does not move, there is said to be a moment at the axis of rotation which resists the rotation.

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For static systems (systems in which the components are at rest), the principle of moments states that the sum of moments within the system must be equal to zero.

Note: To help clarify this idea, if the sum of moments were not zero, then the object in question would be actively rotating.