There are times when momentum is not *exactly* conserved,
but it is still useful to get a rough idea for the result
of a complicated process.

Example:A big truck travelling at 60 mph southwards collides with a minivan going 55 mph northwards. Which way does the mangled wreckage slide afterwards?

The natural choice for "system" here is
"the truck and the minivan".
If we try to include a significant chunk of time before
and after the collision, then we have to deal with
**external** forces of friction on the tires of the vehicles,
air resistance, and so forth. The wreckage will
eventually slide to a halt, due to friction,
and all the momentum will disappear (it really
ends up being transferred to the ground).

But if we focus in on the collision itself, looking at a narrow slice of time just before and just after,

then momentum will be ALMOST exactly conserved:
over this small interval, the **external** forces
(friction, air resistance) are much less important
than the **internal** forces
between the two vehicles.

Example:A big truck of massM = 10 tonstravelling at 60 mph southwards collides with a minivan of massm = 1 tongoing 55 mph northwards. Which way does the mangled wreckage slide afterwards, and how fast does it initially move?

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.