# Questions about conservation of momentum

by sgstudent
Tags: conservation, momentum
 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
 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.
P: 645
 Quote by Khashishi 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.
Oh, you mean the change in momentum is equal to the external force multiplied by the duration of it?

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

Thanks :)