Momentum Is Conserved Even When Energy Is Not

Lab Brief #3

🎯 Your mission

Predict it first. Then prove yourself right (or wrong).

⚡ The twist

Energy never disappears — it just hides somewhere else.

What you'll learn

Total momentum p = mv is unchanged in any collision, because the two bodies exert equal and opposite forces on each other for exactly the same length of time. A 2 kg trolley at 3 m/s striking a stationary 1 kg trolley and sticking to it carries 6 kg m/s into a combined 3 kg, so the pair leaves at 2 m/s. Kinetic energy is not conserved in that collision — some is dissipated as heat and sound — which is why momentum, not energy, is the reliable bookkeeping tool.

Key words

momentum
Mass multiplied by velocity, measured in kg m/s, and conserved in every collision.
inelastic collision
A collision in which the objects stay together and kinetic energy is dissipated, while momentum is still conserved.

Make a hypothesis first

Before you start: which will move faster, and why?

Say your guess out loud — then find out if you were right.

Get ready

🧰 What you need

two dynamics trolleys
level track
slotted masses
motion sensor or phone camera

Get ready

⚠️ Safety first

Keep fingers out of the collision point and stop the trolleys before the end of the track.

Step 1 of 4 · Read

Think about what you just learned: say what "momentum" and "inelastic collision" mean in your own words before you start.

Step 2 of 4 · Do

Run two trolleys into each other on a level track, one loaded and one empty, and record their masses and velocities before and after with a motion sensor or video. Calculate total momentum before and after, then calculate total kinetic energy before and after and compare which one balanced.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

A 2 kg trolley at 3 m/s hits a stationary 1 kg trolley and they stick together. Their combined velocity is:

How sure are you?😐 maybe

Question 2 of 3

In an inelastic collision, which quantity is still conserved?

How sure are you?😐 maybe

Question 3 of 3

Momentum is conserved in a collision because the forces on the two objects are:

How sure are you?😐 maybe

Mind = blown

⚡ Static electricity is the same physics as lightning, just smaller.

Nice work!

You got 0 of 3 right.

Next: Drag Grows with the Square of Speed All lessons

🎯

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✨ More to explore

🌍 Where you see this in real life: You use this every time you flip a light switch, ride a bike, or kick a ball.

🏆 Stretch challenge (try it this week): Time how long a small ball rolls down something, then change the slope and try again.

👨‍👩‍👧 For the dinner table: “Show me something at home that uses this kind of energy.”

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Momentum Is Conserved Even When Energy Is Not

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Momentum Is Conserved Even When Energy Is Not

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