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A simple answer would be that after working out the momentum transfers after a collision for instance, you can calculate the kinetic energy that is left. The kinetic energy that is left however is not necessarily all the energy that is left, because energy can be present in other forms after a collision, only elastic collisions have a conservation of kinetic energy also.
If we don't know where the energy after a collision has disappeared, automatically we assume it must be present in another form, but then we are using the law that energy is conserved, instead of deducing it from our measurements.
To measure empirically the conservation of energy we need to measure a lot more than just the kinetic energy of the moving bodies, like their temperature, changes in chemical structure and energy.
If we don't know where the energy after a collision has disappeared, automatically we assume it must be present in another form, but then we are using the law that energy is conserved, instead of deducing it from our measurements.
To measure empirically the conservation of energy we need to measure a lot more than just the kinetic energy of the moving bodies, like their temperature, changes in chemical structure and energy.
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Simple:
momentum can have an elastic collision (when they bounce off each other). in this case ( when they bounce off each other) energy is conserved. But in a car crash which is a collision, energy is not conserved. this is because energy is lost to other sources like sound and heat. and plus, momentum is ALWAYS supposed to be conserved in a collision.
momentum can have an elastic collision (when they bounce off each other). in this case ( when they bounce off each other) energy is conserved. But in a car crash which is a collision, energy is not conserved. this is because energy is lost to other sources like sound and heat. and plus, momentum is ALWAYS supposed to be conserved in a collision.