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How does one calculate Momentum of an object? 
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#1
Jan2609, 05:30 PM

P: 39

Ok momentum is not KE, but one can calculate the amount of KE stopping the object and measureing how high it raised a weight, or now much frictional heat it generated. If momentum would be a different number, then how do you measure it?
What is momentum? KE will also travel in the same direction until a force is acted upon it, so what is the purpose of having a term called momentum? 


#2
Jan2609, 09:06 PM

P: 168

Momentum is mass times velocity, typically written p=mv. Kinetic Energy is mv^2/2. In any interaction between two masses momentum is always conserved. Energy is conserved also, but it can change forms from kinetic energy to heat energy for example. Or the kinetic energy can be stored in a spring or as potential energy. So even though energy is always conserved, it does not have to be conserved as kinetic energy. But regardless of the interaction momentum is conserved.



#3
Jan2609, 11:54 PM

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P: 11,782

Another difference between momentum and KE is that momentum is a vector (has magnitude and direction) whereas KE is a scalar (magnitude only).



#4
Jan2709, 02:43 AM

P: 39

How does one calculate Momentum of an object?



#5
Jan2709, 06:15 AM

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P: 11,782

When talking about conservation of momentum in this case, you have to include the momentum of whatever the spring is attached to. If the spring is effectively attached to the Earth, then when the object collides with the spring and compresses it, the Earth recoils with a very very very very small velocity because of its very very very very large mass.



#6
Jan2709, 06:27 PM

P: 39




#7
Jan2709, 06:30 PM

Sci Advisor
P: 2,193

Momentum is conserved, because as it has been indicated, momentum is a vector quantity. To put a coordinate system on your example, two objects of equal mass flying in space. If one is flying in the +x direction, and the other in the x direction, the total momentum of the system is exactly zero. So, when they hit the spring, we expect momentum to remain the same (zero), and it is.



#8
Jan2709, 06:37 PM

P: 39




#9
Jan2709, 06:37 PM

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#10
Jan2709, 06:48 PM

Sci Advisor
P: 2,193

I'm not sure if the kinetic energies necessarily have to be the same, though. (This would mean that the distance the force of one objcet on the other is applied through is the same for both objects, and this doesn't necessarily seem intuitive to me.) 


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