A train is harder to stop than a car moving at the same speed because the train has more momentum than the car. This is because of its greater mass. If we go back to the definition of force: F = m. a

Acceleration is rate of change of velocity

From combining all the three above equations, we can say that force is rate of change of momentum. Another term that is frequently used in equations of motion is impulse. Impulse is force multiplied by time. Impulse has the same units as momentum (newton – second or kilogram–meter per second), but there is a distinction: impulse is applied force at an instant of time. We will discuss about impulse a little later.

To fully describe the momentum of a 1 kg ball moving eastward at 5 m/s, you must give information about both the magnitude and the direction of the ball. It is not enough to say that the ball has 5 kg. m/s of momentum; the momentum of the ball is not fully described until information about its direction is given. The direction of the momentum vector is the same as the direction of the velocity of the ball.

If the ball is moving eastward, then its momentum can be fully described by saying that the ball has a momentum of 5 kg. m/s, directed eastward. From the definition of momentum, it becomes obvious that an object has a large momentum if either its mass or its velocity is large. Both variables are of equal importance in determining the momentum of an object. An object having zero velocity has zero momentum.