Momentum is how we track motion when things interact. And it’s not just for car crashes and sports, it’s a conserved quantity, meaning it stays the same, no matter what. Let’s break it down.

What Is Momentum?

Momentum is the product of how much stuff you have and how fast it’s moving.

Momentum (p) = mass × velocity

p = mv

Where:

• p = momentum
• m = mass
• v = velocity

It’s a vector, meaning it has both size and direction. So changing either mass or velocity changes the momentum.

The heavier or faster something is, the more momentum it carries. A bowling ball rolling slowly has more momentum than a ping pong ball launched at full speed. Why? Mass matters.

Impulse

Momentum doesn’t stay the same if a force acts over time. That’s where impulse comes in.

Impulse = Force × time

J = F(t)

And impulse is equal to the change in momentum:

F(t) = Δp

This means if you want to change something’s momentum, speed— it up, slow it down, or reverse it — you need to apply a force for some amount of time.

Think:

• Bouncing a basketball = short impulse
• Airbags in a car crash = longer impulse to reduce force
• Jumping off a table and bending your knees = same momentum change, less injury

Collisions

When two objects hit, momentum gets transferred, but it’s not destroyed. This is called the Law of Conservation of Momentum:

The total momentum before a collision = the total momentum after a collision (as long as no outside forces interfere)

There are two main types of collisions:

1. Elastic Collisions

• Objects bounce off each other
• Both momentum and kinetic energy are conserved
• Happens in idealized scenarios (like gas particles)

2. Inelastic Collisions

• Objects stick together or deform
• Momentum is conserved, but kinetic energy is not
• Some energy becomes heat, sound, or damage

Most real-life collisions are partially inelastic, some bounce, some stick, some break.

Real Examples

• Car crash → Inelastic (total kinetic energy drops, but momentum is still tracked)
• Billiard balls → Nearly elastic
• Jumping on a skateboard → Combined momentum must stay the same
• Running into a wall → You lose momentum, but the wall gains basically none because of its huge mass

Even the Earth moves slightly when you jump. (You just don’t notice it because it’s so massive.)

Why This Matters

Momentum explains:

• Why hitting a wall hurts more than hitting a pillow
• How rockets push off their own fuel and fly
• How figure skaters spin faster when they pull their arms in
• Why bulletproof vests spread out the force of a projectile

In physics, interactions are never just chaos. There’s an order. A math. A law. Even the most violent collisions follow strict rules.

Momentum is basically motion’s fingerprint, and once you realize that it’s conserved, even during the most chaotic crashes, you’ll start to see that even in impact, physics keeps its balance.