and PY, ure 42, right?, you should know better than thinking momentum and kenetic energy are the same, its completly different thing

Where did I say they were the same thing?

Momentum is Mass times velocity

Kinetic Energy is 1/2 MV squared

Whoopdy bloody do!

What I said was

(For simplicity's sake momentum and kinetic energy are assumed to be the same)

That just means that is a shit load easier to discuss and explain things in terms of conservation of momentum that it is to bring KE into the conversation.

And

**NO**! Momentum most certainly does not alway equal zero!

Try throwing a ball at a wall covered in soft plaster.

Try this calculation for size!

Before collision

Ball velocity = 20

Ball mass = 10

Ball Momentum = MV = 200

Wall mass = about 20,000

Wall velocity = 0

Wall momentum = 0

Collision!!!!!!!!!

!!!!!!SPLAT!!!!!!!

Ball sticks in soggy plaster

Ball velocity now zero

Ball momentum now zero

Wall velocity OOH Look It's still ZERO

Wall momentum is STILL ZERO

Where did your momentum go?

Well neither the wall or the ball are moving any more so IT ISN'T THERE ANY MORE

Why?

Because the kinetic energy of the ball was absorbed by the plaster and used up completely in reshaping it from a nice flat surface into a big sodding dent!

KE and momentum may not use the same calculation but they are very similar in nature. Both can be imparted or absorbed by other features in any system and DO NOT ALWAYS EQUAL ZERO. In fact the only time when momentum in a system can actually be zero is when that entire system has reached maximum entropy.

That is just basic physics.