Momentum refers to moving things. It is a product of the mass of an object
and its velocity. Momentum is why the driver of a car applies the brake
to stop the car rather than just taking his foot off the accelerator. The
car has gathered momentum and will continue to move forward after the driver
stops accelerating it.
greater the mass of the object, the more the momentum. It is harder to
stop a large tractor trailer than it is a small compact car. Freight trains
take much longer to stop than a short passenger train.