Forces and Motion | Energy and Thermal Physics

The motion of an object

Physics Narrative for 14-16 Supporting Physics Teaching

Measures of motion and how the force acting can change them

Alice, Bob or Charlie can record the velocity of an object. Combining these measures with the mass of the object (invariant – so the same for all three) results in two measures of motion for any object.

Energy in the kinetic store (a scalar quantity):

energy in the kinetic store = 12mv 2

Momentum (a vector quantity): p = m × v.

Evidently these measures depend on the point of view chosen for the recording of the velocity, so won't be invariant. Alice, Bob and Charlie will calculate different values for the quantities unless they happen to all be moving together, so having the same velocity.

All kinds of compound things can be taken as objects, so long as we don't have to consider the internal structure. That is, any internal interactions need not be considered for the purpose we have at hand, and so the things can be reduced to point masses.

You can augment either quantity by acting on the object with a force, and both are compensated quantities (one case for each is given here). energy = large force × small distance momentum = large force × short duration or: energy = small force × long distance momentum = small force × long duration


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