Momentum: quantity of motion
Early physicists found that in collisions between two objects the sum of the products of
each object's mass and velocity before the collision is equal to the same sum after the
collision, even though the velocities change. This quantity was given the name
momentum. It turns out that it is a very important physical quantity.
The symbol for momentum is "p", and it is a vector. Again, an object's momentum is the
product of its mass and its velocity:
p = mv
Using SI units, the units of momentum must be kg x m/s, or just kg∙ m/s. It does not have
a special name, the way force has its own special unit, the newton.
Momentum is an important quantity when it comes to collisions. For example, when a
bat hits a ball, the momentum that the bat loses is equal to the momentum that the ball
The more momentum something has, the more difficult it is to stop it. (The more inertia
it has, the more difficult it is to accelerate or decelerate it.)
Unitmania: Since a newton is equal to a kg∙ m/s2, a newton times a second is equal to a
kg∙ m/s. So, another way of giving the unit of momentum is N∙s, called a “newton
1. What is the momentum of a 5 kg rock falling downward at 20 m/s?
2. What is the momentum of a 2.0 x 10-3 kg insect flying east at 4 m/s?
3. Which has greater momentum, a 1000-kg car driving at 30 m/s, or a 3000-kg truck
driving at 10 m/s?
4. Name the two different ways of expressing the units of momentum.
5. What is the definition of momentum?
6. Does a falling object's momentum increase as it accelerates downward?
7. Find p for a 4 kg object moving at 7 m/s.
8. We decide to make the positive x-direction to the right, and negative to the left. How
much momentum does a 30-kg cart moving at 2 m/s to the left have?
9. Newton originally wrote his 2nd law in terms of momentum: F = p/t. Show how
this is equivalent to the more common expression F = ma.