Momentum and collisions

Momentum and collisions are closely related in physics. Keep reading to find out how.

Collision occurs of course when objects collide. We will focus on the collisions of two objects in this lesson.

When two objects collide in the absence of external forces, the net momentum of the objects before the collision is equal to the net momentum of the objects after the collision. 

Mathematically, we can write the equation below

net momentum (before collision) = net momentum (after collision)

Remember that momentum = mv

Therefore, we can write the following equation.

net mv (before collision) = net mv (after collision)

There are two types of collisions. The first type is elastic collisions and the second type is inelastic collisions.

Elastic collisions:

When a collision occurs between two objects and the objects are not permanently deformed and no heat is generated, we say that the collision is elastic. The word elastic probably came from the fact that when you stretch an elastic band, it will always go back to its original shape instead of being deformed.

Elastic collisions does not occur perfectly since some heat is usually generated during collisions. 

To solve problems involving elastic collisions, we will need to assume that the heat generated is negligible. We can also use two marbles since marbles are not usually deformed permanently after collision. We can consider a couple of cases. The two cases below assume that the masses of the marbles are equal. The other important thing to keep in mind is that no momentum is loss or gain during the collision. Instead, the momentum is simply transferred.

Case #1: Blue marble is moving toward green marble while green marble is at rest.

Elastic collisions

Notice that after collision, the blue marble is at rest while the green marble is now moving.

Case #2: Blue marble and green marble are moving toward each other.

Elastic collisions

Notice that after collision both marbles move away from each other

Inelastic collisions:

In inelastic collisions, the objects colliding will be deformed or distorted.

However, after they collide, they can either blend together to form a single unit or they can bounce off each other.

This usually happens during automobile accident. Sometimes, one car hits another car, gets stuck to that car, and keep pushing it for a little while before coming to a full stop. Other times, the collision is head-on and the cars bounce off.

Now you are ready to solve some momentum and collisions word problems.

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