Question

How can you decrease the momentum of an object?

Answer 1

The momentum of an object is the product of (its mass)
and (its velocity). So, if you want to decrease the object's
momentum, then you're going to have to decrease one of
those quantities.

You have exactly two choices:

1). Reduce its mass. (Put the object on a diet, or cut a piece off.)
or
2). Reduce its speed.

Answer 2

You will have to "put force against the object to slow it down." Momentum is the force that is keeping a object moving in a certain direction so if you would want to slow down the object you will have to put another force against the object to slow it down or stop it. For example: a person kicks a ball, the ball moving is the momentum. So, if you would want to stop the ball you would have t put something in its path to slow it down which is the decreasing of it's momentum. Therefore you would put a bump in the wall and when the ball hits the bump it slows down.

Hope this helps!