Physics for Kids

Momentum and Collisions

Momentum is a measurement of mass in motion. Any object that is moving has momentum. In physics, momentum of an object is equal to the mass times the velocity.

Momentum is usually abbreviated using the letter "p" making the equation look like:

p = m * v

where p is the momentum, m is the mass, and v is the velocity.

From this equation you can see that both the velocity of the object and the mass have an equal impact on the amount of momentum. You have more momentum when you are running than when you are walking. By the same token, if a car and bicycle are traveling down the street at the same velocity, the car will have more momentum.

Momentum is typically measured in kilograms times meters per second (kg*m/s) or newton-second (N s).

Because velocity is a vector, momentum is also a vector. This means that in addition to the magnitude of momentum (which is given by p = m * v), momentum also has a direction. The direction of momentum is shown by an arrow or vector.

When two objects bump into each other, this is called a collision. In physics, a collision doesn't have to involve an accident (like two cars crashing into each other), but can be any event where two or more moving objects exert forces on each other for a short period of time.

Examples:

- One ball striking another on a pool table
- A baseball bat hitting a ball
- Your fingers striking the key on the keyboard

An important theory in physics is the law of momentum conservation. This law describes what happens to momentum when two objects collide.

The law states that when two objects collide in a closed system, the total momentum of the two objects before the collision is the same as the total momentum of the two objects after the collision. The momentum of each object may change, but the total momentum must remain the same.

Example:

If a red ball with a mass of 10 kg is traveling east at a speed of 5 m/s and collides with a blue ball with a mass of 20 kg traveling west at a speed of 10 m/s, what is the result?

First we figure out the momentum of each ball before the collision:

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 standing still has a momentum of 0 kg m/s.

- No one is quite sure why "p" is used for momentum. It likely came from the Latin word "petere" which means "go towards". They couldn't use "m" because that was already used for mass.
- A change in momentum is called impulse.
- An elastic collision is one in which no kinetic energy is lost.
- An inelastic collision is one in which some of the kinetic energy of the colliding bodies is lost. This is because the energy is converted into another type of energy like heat or sound.
- The recoil of a gun is because of the conservation of momentum. The gun moves back at a lower velocity than the bullet because of its greater mass.

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