Asked • 07/11/19

Describe the difference between moment and momentum in physics.

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Mark H. answered • 07/11/19

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These are 2 radically different principles--used in different ways. The biggest thing they have in common is the first six letters!!


Momentum applies to objects in motion and is the product of mass and velocity. It is not the energy, but the variables are the same.

Where momentum is m*v, kinetic energy is (m*v^2)/2 We learn that momentum is always conserved, but energy is conserved only in a special case (which can be approached, but never fully attained)


By contrast, "moment" is an expression of the "rotational force" caused by a force acting at some distance from a fulcrum. Moment is also called "torque", and is a static term rather than an expression of any kind of energy.

Just as pushing on a stationary object does not consume energy, attempting to turn a stationary object gives the same result.

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Michael D.

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Hmm beg to differ as I recall moment is usually a geometric quantity related to the inertia of an object to an applied Torque. Just as mass is the inertia of an object to an applied translational force. We have F=ma and T= Ia correlations. We have moment arms with regard to levers and fulcrums and anchoring beams to walls or building bridges and mapping stresses. We also have translational momentum inertia scaled by velocity and rotational momentum the moment of inertia scaled by angular velocity. Not sure about your use of energy....pushing on a stationary object does not do any work on the object, it certainly consumes calories. Work is also Torque multiplied by distance moved in parallel. Torque usually does no work as the wheels on shafts don't move in the direction of torque....but elastic materials can twist and work is done by Torque to change that potential energy. Neither Translational or rotataional forces are energy terms. The inertia in the force fields does scale the change in potential energy. Finally momentum is only conserved when the external forces in the process sum to zero. This is Newtons first law ( No external forces no change in the motion of the system) using dP/dt =F we see momentum is not conserved for impulses to the system...We call this the impulse force...and for very short time durations state delta momentum is Force multiplied by the time pulse. Example a bat hitting a ball where both momentum and mechanical energy are not conserved in the inelastic collision. Finally in space-time we have some real issues in defining inertia but in simplicity it is the resistance of the object to move into a higher energy geodesic from its current geodesic. Which is how we start relating mass (inertia) to relativistic energy as speeds near that of the photon. LIGO now is an observation of a relativistic bat hitting a relativistic ball ,and we don't know what to say about that physics.
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07/12/19

Mark H.

The one thing I see in there is the definition of "moment" that I did not cover--"moment of inertia." As you say, it is the rotational equivalent to inertia. But....I do not agree with some other statements. For example, I thought momentum was always conserved. Could you post a reference that explains what the exceptions might be?
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07/12/19

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