Smashes keep momentum, audit energy

Science · Forces & Motion · ages 17-18

Name ______________________ Date ____________

What defines an elastic collision?
- The pair stick together
- It keeps all its kinetic energy
- Total momentum doubles
What is always conserved in a collision with tiny outside forces?
- Momentum
- Kinetic energy
- Speed
Kinetic energy is conserved in every collision.
Circle one: True False
Two trolleys hold 18 J of kinetic energy before impact and 0 J after, stuck together. How do you classify the crash?
- Elastic
- Perfectly inelastic
- An explosion
Right is positive. A 2 kg trolley moves at 3 metres per second and a 2 kg trolley moves at minus 3. What is the total momentum before impact?
Answer: ______________
An inelastic crash ends with less kinetic energy. Where did the missing part go?
- It heated metal and made sound
- It vanished from physics
- It turned back into speed
Tom totals a crash, finds less kinetic energy after, and says energy was destroyed. What is his error?
- He added the totals wrong
- Momentum vanished too
- Energy cannot vanish, it changed form
A crash holds 18 J of kinetic energy before and 0 J after. How many joules never came back?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

It keeps all its kinetic energy · Elastic means kinetic energy survives the hit untouched, alongside momentum.
Momentum · Total momentum balances in every crash, elastic or not.
False · Only elastic hits keep it all. Inelastic ones lose some to heat and sound.
Perfectly inelastic · The totals shrank and the pair share one velocity, which is the extreme inelastic case.
0 · Add with signs: 6 plus minus 6 is 0.
It heated metal and made sound · Lost energy changed form into heat, sound, and permanent bending.
Energy cannot vanish, it changed form · The gap is real but it lives on as heat, sound, and dents, not as nothing.
18 · The gap between the totals is the price: 18 minus 0 is 18 joules.