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Momentum in Physics is a measure of the amount of motion an object has. If an object is moving then it is said to have momentum.

Momentum definition
Every object has a certain mass and Momentum in physics can also be defined as ‘mass in motion’.

The amount of motion an object has depends on two factors:

  • How much mass is moving and
  • How fast is this mass moving also referred to as the velocity of the object

Therefore, momentum depends on two variables mass and velocity. The equation can be stated as:
Momentum = mass * velocity

                                         

Momentum in Physics | Image source: youtube.com

Thus, momentum of an object is equal to the mass of the object times the velocity of the object. The symbol for momentum is p. Therefore, symbolically the equation can be written as:

P = m * v   where m is the mass and v is the velocity

The standard metric unit of momentum is mass units times velocity units i.e. kg*m/s. The momentum vector points in the direction of velocity.

When an object is in motion it will take some effort to stop the object. If the object has a lot of momentum then it is going to be hard to stop. Thus, more the momentum of an object the more difficult it is to stop.

Momentum is directly proportional to the mass of the object as well as its velocity. The object will have large momentum if both mass and velocity of the object are large.

For example if a truck and a cycle were to go at the same speed on the road then the momentum of the truck will be greater as it has greater mass. When an object is at rest, its momentum is zero.

Did you know: If there is no force acting on a system, then its momentum will be conserved.

The law of momentum conservation
The law states that, “If two objects collide in an isolated system, the total momentum of the objects before the collision will be equal to the total momentum of the objects after the collision.”

Note: An isolated system here refers to a system that cannot exchange matter or energy outside its boundaries.

The total momentum is a constant and unchanging value. This can be explained with the following example.

Consider two balls - Ball 1 and Ball 2 that collide and the forces acting on the two balls are equal in magnitude but opposite in direction. The force equation can be written as:
F1 = - F2

The forces act on the balls for a given amount of time. The time for which the force acts upon the first ball is equal to the time the force acts upon the second ball. The time equation can be stated as:
t1 = t2
 
If the forces acting upon the balls are equal in magnitude and opposite in direction and the time for which the forces act are also equal then naturally the impulse experienced by the two is also equal in magnitude and opposite in direction. The impulse equation can be stated as:
F1 * t1 = - F2 * t2

The impulse experienced by an object is equal to the change in momentum of that object. Since the impulses are equal and opposite, the change in momentum of the balls will also be equal and opposite. The change in momentum equation can be stated as:
m1 * Δv1 = - m2* Δv2

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