# Conservation of momentum

1. Jan 29, 2010

### hangover

Conservation of linear momentum holds if no external net force acts on the system. Therefore we use them to calculate the velocities of objects after collison.
However, my professor said that we can use the princple of Conservation of linear momentum to estimate the velocities of objects when there is external net force acting.
For example, pulling a string which is connected with a block. No friction and string has mass and it is loose at first. We can estimate the "initial" velocity of the block by Conservation of linear momentum:
(Mass of string)times(velocity string)=(Mass of block+Mass of string)times(initial velocity block)
He said that the conservation of linear momentum holds for an infinismal of time.
It is because Ft=mv-mu, when t is very small, even though there exists F, no impulse, then the momentum is conserved.
Is that right? Also interestingly, the answer of this approach is quite similar to that considering the block as individual objects,then F=ma, then v=at, then take t as 1sec.
Thx