Collisions conserve momentum. Whether they conserve kinetic energy depends on type.
Collision lab — watch momentum survive the crash
Momentum before
115
Momentum now
115
Kinetic energy before
5213
Kinetic energy now
5213
Watch the bars. Momentum is conserved in every collision.
Walk through
Step 1 of 4
Set up
Pick a direction as positive. Write each object's momentum p = m·v *before* the collision, keeping signs.
Conservation of momentum
Momentum p = m·v is a vector — direction matters. Total stays constant when no external force acts.
Your turn
A 2 kg cart at 3 m/s hits a stationary 1 kg cart and they stick together. Final speed?
Crumple zones in cars are *deliberately* inelastic — they stretch the collision out in time so the force on you is smaller.
Two types
- Elastic — KE conserved (billiard balls, atomic).
- Inelastic — KE lost to heat, sound, deformation (cars).