What is Momentum?
Momentum is a measure of mass in motion. Any moving object has momentum. In physics, the amount of motion an object is equal to mass times speed.
momentum = mass * velocity
Momentum is often abbreviated with the letter “p”, making the equation look like this:
p = m * v
where p is the momentum, m is the mass, and v is the velocity.
From this equation, you can see that the speed and mass of the object have the same effect on momentum. You are more motivated when you run than when you walk. Likewise, if a car and a bicycle are traveling on the road at the same speed, the car will have more momentum.
How to Measure Momentum
Momentum is usually measured in kilograms multiplied by meters per second (kg*m/s) or newton-seconds (N s).
Momentum is a Vector
Since velocity is a vector, momentum is also a vector. This means that in addition to the magnitude of momentum (given by p = m * v), momentum has direction. The direction of momentum is indicated by an arrow or a vector.
Collisions
When two objects collide, it is called a collision. In physics, a collision is not necessarily an accident (like two cars crashing into each other), but can be any event in which two or more moving objects exert force on each other in a short time.
Examples:
A ball hits another ball on the pool table
A baseball bat hits a ball
Your finger presses a key on the keyboard
Collisions and the Conservation of Momentum
An important theory in physics is the law of conservation of momentum. This law describes what happens to momentum when two objects collide.
The law states that when two bodies collide in a closed system, the total momentum of the two bodies before the collision is equal to the sum of the momentum of the two bodies after the collision. The momentum of each object can change but the total momentum must remain constant.
Example:
If a red ball of mass 10 kg is moving eastward at 5m/s and collides with a blue ball of mass 20kg moving westward at 10m/s, what is the result?
We first determine the momentum of each ball before impact:
Red ball = 10 kg * 5 m/s = 50 kg m/s east
Blue ball = 20 kg * 10 m/s = 200 kg m/s west
The resulting momentum will be:
Both balls = 150 kg m/s west
Note: An object at rest has an impulse of 0 kg m/s.
Interesting Facts about Momentum and Collisions
No one really knows why “p” is used for momentum. It probably comes from the Latin word “petere” which means “to go towards”. They can’t use “m” because it’s already in use for the masses.
A change in momentum is called an impulse.
An elastic collision is a collision with no loss of kinetic energy.
An inelastic collision is a collision in which part of the kinetic energy of the colliding bodies is lost. This is because the energy is converted into another type of energy such as heat or sound.
The recoil of the gun is due to conservation of momentum. The barrel recoils at a lower velocity than the bullet due to its greater mass.