Energy & Momentum

🎯 What Are Energy and Momentum?

Anything in motion — a rolling ball, a speeding car, a thrown baseball — carries two important physical quantities: momentum and kinetic energy. They're related, but they're not the same thing, and telling them apart is the key to understanding collisions.

🎯 Momentum

Momentum measures how hard it is to stop something that's moving. It depends on both how much mass is moving and how fast it's going.

Momentum has a direction — a car moving north and an identical car moving south at the same speed have momentum pointing opposite ways. That's why we use positive and negative numbers for velocity: positive for one direction, negative for the other.

⚡ Kinetic Energy

Kinetic energy measures the energy of motion — how much "work" that moving object could do if it hit something. It depends on mass too, but speed matters a lot more, since it's squared.

Because velocity is squared, doubling an object's speed doesn't just double its kinetic energy — it quadruples it. That's a big part of why high-speed crashes are so much more dangerous than slow ones.

💥 Types of Collisions

No matter what kind of collision happens, one rule never breaks: total momentum is always conserved — the combined momentum right before a collision always equals the combined momentum right after. Kinetic energy is a different story.

🔄 Elastic Collisions

In an elastic collision, objects bounce off each other and both momentum and kinetic energy are conserved — none of the motion energy is lost to heat, sound, or bending metal. Real elastic collisions are rare, but some come very close.

• Billiard balls clicking together on a pool table
• Two marbles bouncing off each other
• Gas molecules bouncing around in the air

🧲 Inelastic Collisions

In an inelastic collision, momentum is still conserved, but kinetic energy is not — some of it converts into heat, sound, or the energy of bending and crumpling. In a perfectly inelastic collision, the objects stick together and move off as one.

• A football tackle, where both players end up moving together
• Two train cars coupling together
• A car crash, where crumpling metal absorbs energy

♻️ Conservation of Energy

The law of conservation of energy says energy can never be created or destroyed — only changed from one form into another. When a roller coaster car climbs a hill, it gains potential energy (stored energy due to height). As it races back down, that potential energy converts into kinetic energy (energy of motion).

A swinging pendulum shows the same trade constantly: kinetic energy at the bottom of the swing, potential energy at the top, back and forth. A bouncing ball never quite returns to its starting height because some of its energy converts into heat and sound with every bounce — the total energy is still conserved, it's just spread out into forms that are harder to use.

📖 Energy & Momentum Glossary

Momentum

Mass in motion: p = m × v. Has a direction, and is always conserved in a collision.

Kinetic Energy

The energy of motion: KE = ½ × m × v². Only conserved in elastic collisions.

Potential Energy

Stored energy an object has because of its position or shape, like height or stretch.

Conservation of Momentum

Total momentum before a collision always equals total momentum after.

Conservation of Energy

Energy can't be created or destroyed — only changed from one form to another.

Elastic Collision

A collision where kinetic energy is conserved and objects bounce apart.

Inelastic Collision

A collision where kinetic energy is lost, often because objects stick together.

Mass

The amount of matter in an object, measured in kilograms (kg).

Velocity

Speed in a specific direction, measured in meters per second (m/s).