ScientistGrades 10–12⏱ 20 min

Momentum and collisions

Crash two carts on a track. Bouncy or sticky, the total momentum before equals the total after.

Standards: HS-PS2-2HS-PS2-3

1Wonder

A bowling ball rolls slowly into the pins and sends them flying. A ping-pong ball thrown much faster barely moves them. What does the bowling ball have more of?

2Explore

Collision lab

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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.

Total momentum 3 kg·m/s Kinetic energy 4.5 J

Mass of blue cart m₁ 2 kg Velocity of blue cart v₁ 2 m/s Mass of red cart m₂ 1 kg Velocity of red cart v₂ −1 m/s

Collision

Try this

Press Play and watch the purple total arrow during the crash. Does it change?
Set both carts to 1 kg, choose Bouncy, and send the blue cart into the red one at rest. What happens?
Choose Sticky. Does the total momentum change? What about the kinetic energy?
Make a heavy cart hit a light one, then a light cart hit a heavy one. Which one bounces back?

3Explain

Step 1 of 5

Momentum is mass times velocity

Momentum is p = m × v. A slow truck can have far more momentum than a fast baseball because its mass is so much bigger.

Conservation of momentum

\htmlClass{c-blue}{m_1 v_1} + \htmlClass{c-red}{m_2 v_2} = \htmlClass{c-blue}{m_1 v_1'} + \htmlClass{c-red}{m_2 v_2'}

When no outside force acts, the total momentum before a collision equals the total momentum after. Kinetic energy is kept only in perfectly bouncy (elastic) collisions.

4Practice

Question 1 of 5

What is the momentum of a 1500 kg car moving at 20 m/s?

5Recap

Momentum p = mv. It has a direction, so it can be negative.
In a collision, the carts push each other equally and oppositely, so their momentum changes cancel.
With no outside force, total momentum is conserved in every collision.
Bouncy (elastic) collisions also keep kinetic energy; sticky ones lose some to heat and sound.