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Questions about conservation of momentum 
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#1
Jan3013, 09:05 PM

P: 645

What does having no external force on a closed system mean? For example if I have 2 objects colliding. One travels at a constant speed while the other travels with a constant acceleration. In this case is an external force being applied on the system?
If so, only the m1u1+m2u2=m1v1+m2v2 can be used if the two objects have a constant velocity? Also, during the collision won't there be a force being applied on the object? So how would momentum be conserved? During the collision the formula F=change in mv/time is used. So is the conservation only after this collision? Thanks for the help 


#2
Jan3013, 09:16 PM

P: 886

An object can't accelerate without some applied force. Momentum isn't conserved if you have an external force on the system. Actually, the change in total momentum is equal to the external force.



#3
Jan3013, 09:27 PM

P: 645

So if there is an external force, how will the collision be like? Or are we unable to solve for the final velocities. Thanks :) 


#4
Jan3013, 09:33 PM

Sci Advisor
P: 1,909

Questions about conservation of momentum
During the collision, you're certainly right that there are forces involved. But Newton's third law implies that the total change in momentum due to a collision is zero. So the vectorial sum of the momenta of all the objects is the same before and after the collision. (Assuming once again that the only interaction is collisions). 


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