Crash Lab

Smash two carts together! Choose their masses, speeds and bumpers and see what happens to momentum and energy in the crash.

Bumpers
A after
–
B after
–
Momentum before
1.00 kg·m/s
Momentum after
–
KE before
1.25 J
KE after
–

The maths, live

Momentum before p = mA·uA + mB·uBp = 1 × 1.5 + 1 × -0.5 = 1.00 kg·m/s
Momentum after p = mA·vA + mB·vBCrash them to find out!
Kinetic energy KE = ½mA·v² + ½mB·v²1.25 J before
Bounce e = speed apart ÷ speed togethere = 1

Momentum is the same before and after every crash. Is kinetic energy? Try sticky bumpers and see where the energy goes.

How it works

Momentum is mass times velocity: p = m × v. Velocity has a direction, so a cart moving left has negative momentum. In a crash the carts push on each other with equal and opposite forces, so the total momentum is exactly the same before and after.

Kinetic energy is different. Bouncy bumpers give it all back (an elastic collision). Squashy and sticky bumpers turn some of it into heat, sound and squashing, so there is less kinetic energy afterwards. With sticky bumpers the carts move off together.

Try this

  1. Crash two equal carts with bouncy bumpers, one moving and one still. What happens?
  2. Make cart A three times heavier than cart B. Which cart changes speed more?
  3. Use sticky bumpers. How much kinetic energy is lost? Is any momentum lost?