Momentum

Momentum is the tendency of a moving object to keep moving. You can find the momentum of an object by multiplying its mass times its velocity. In other words, heavy things, and things going fast, have a lot of momentum. A heavy thing that is also going fast have LOTS of momentum. Things with a lot of momentum have a hard time slowing down.

Why should you care? Because trains and large trucks have a LOT more momentum than your car.

Respect the momentum of a truck.
The average freight train takes over a mile to stop.