Simple momentum question

[scilearner]

In this collision

 

 

Before= p=100 kg m/s---> <-----p=120 kg m/s

After= <--combined unit P= 20 kg m/s

 

In this question when I work out the change in momentum for 2 objects it is not the same. When there is a combined unit travelling after the collision can't you find the change in momentum?

[Amr Morsi]

So as not to get confused, make the equation as follows:

Total momentum before=Total momentum after.

 

Note that it is not a must for the change in momentum to be negatively equal. They only moved together.