Conservation of momentum & collisions
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Question
Define momentum.
Answer
**Momentum p = mv** — mass × velocity. It is a **vector** (has direction). Unit: kg m s⁻¹.
Question
State the law of conservation of momentum.
Answer
If no external force acts, the **total momentum before = total momentum after** a collision or explosion.
Question
Is momentum conserved in an inelastic collision?
Answer
**Yes** — momentum is conserved in **every** collision (with no outside force), elastic or inelastic.
Question
What is an elastic collision?
Answer
One where the **total kinetic energy is also conserved** (KE before = KE after). Objects bounce cleanly.
Question
What is a perfectly inelastic collision?
Answer
One where the objects **stick together** and move as one. Momentum is conserved, but the **most kinetic energy is lost** (to heat/sound).
Question
How do you test if a collision is elastic?
Answer
Compare **total KE before** and **total KE after** (E_k = ½mv²). If they're equal, it's elastic.
Question
Why do velocities need + and − signs?
Answer
Velocity has direction — objects moving opposite ways get opposite signs, or the momentum total is wrong.
Question
Two objects stick together — how do you write the 'after' side?
Answer
As **one combined mass** at one common velocity: (m₁ + m₂)v.
Question
Formula for momentum of one object?
Answer
$p = mv$ (given in the data booklet).
Question
Formula for kinetic energy?
Answer
$E_k = \tfrac{1}{2}mv^2$ (given) — used to test elasticity.
Question
In a collision, is kinetic energy always conserved?
Answer
**No** — only in an **elastic** collision. In an inelastic one some KE becomes heat/sound.
Question
Fraction of KE lost when things stick?
Answer
(KE before − KE after) ÷ KE before. It's never zero for a sticking (perfectly inelastic) collision.
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Full study notes for Conservation of momentum & collisions
Topic 1.2 hub
Forces and momentum
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