Why momentum and energy are separately considered?

[libraryismyhome]

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I still don't have a clear sense on the difference and relationship between momentum and energy.
Momentum = mv = Ft
energy = Fd
how come they are not related?? Which one was scalar and which was vector?

34. As a pendulum undergoes damped harmonic oscillation, it loses energy to:

A. momentum. B. wind resistance. B is the best answer. Energy and momentum are altogether different beasts, so energy is neither lost to nor gained from momentum. Choice A should be eliminated. There is no energy lost to torque, because there is no applied angular force, so choice D can be eliminated. Tension is present in the cord, but the loss of energy to tension is questionable. It may be lost to friction in the cord and joint where the cord is attached to the ceiling, but the tension of the cord should not dissipate energy from the system. This eliminates choice C. The best answer is that energy is lost when kinetic energy is transferred through collisions from the moving balls to the gas particles that they strike in their pathway. The best answer is B. C. tension. D. torque.

 

[btc8]

I still don't have a clear sense on the difference and relationship between momentum and energy.
Momentum = mv = Ft
energy = Fd
how come they are not related?? Which one was scalar and which was vector?

34. As a pendulum undergoes damped harmonic oscillation, it loses energy to:

A. momentum. B. wind resistance. B is the best answer. Energy and momentum are altogether different beasts, so energy is neither lost to nor gained from momentum. Choice A should be eliminated. There is no energy lost to torque, because there is no applied angular force, so choice D can be eliminated. Tension is present in the cord, but the loss of energy to tension is questionable. It may be lost to friction in the cord and joint where the cord is attached to the ceiling, but the tension of the cord should not dissipate energy from the system. This eliminates choice C. The best answer is that energy is lost when kinetic energy is transferred through collisions from the moving balls to the gas particles that they strike in their pathway. The best answer is B. C. tension. D. torque.

The way I differentiate them is that momentum is always conserved while energy may or may not be conserved. Energy is most often lost due to friction, resistance, etc.

Basically, I would approach this problem by thinking that since momentum is always conserved, it cannot be lost, and whenever you see resistance (or friction), just think energy loss.

 

[gunj122]

momentum is NOT always conserved. if there are external forces acting (such as gravity, friction, etc), then momentum will not be conserved.

 

[btc8]

momentum is NOT always conserved. if there are external forces acting (such as gravity, friction, etc), then momentum will not be conserved.

Are you sure? Friction would dissipate energy, but not momentum. In a closed system, the law of conservation of momentum states it [momentum] is always conserved. For these instances, such as on the MCAT, I think it's safe to obey the law of conservation of momentum.

 

[Rabolisk]

A damped harmonic oscillation is not a closed system, btc8. What happens to the momentum of the pendulum as time goes on?

 

[Hoya2009]

This has nothing to do with the conservation of momentum. Which is not conserved if an outside force acts on the object in question. See: Impulse.


You can't lose energy to "momentum", "tension" or "torque", those concepts can not be related in that manner. Energy can only be lost through one object/force doing work on the one in question. Only wind resistance makes sense in that context.
Wind resistance, is a force of friction which will dissipate kinetic energy, resulting in heat and decreased amplitude of oscillation.

 

[Blaughable]

Kinetic Energy is a function of momentum 1/2PV+1/2Iomega^2, P=momentum

You lose energy to forces. It just doesn't make sense to lose energy to momentum. Not sure what else to say.