Here's why:

* Mass: The more massive an object is, the harder it is to change its motion. A large truck moving at 10 mph has more momentum than a small car moving at the same speed because the truck has more mass.

* Velocity: The faster an object is moving, the more momentum it has. A car moving at 60 mph has more momentum than the same car moving at 30 mph.

The formula for momentum is:

Momentum (p) = Mass (m) * Velocity (v)