Momentum and collisions — Physics, 14–17 years
Momentum helps us predict what happens when moving objects push, hit or bounce off one another.
The idea
Momentum measures how much motion an object carries. It depends on both mass and velocity: a heavy, slow trolley can have the same momentum as a light, fast ball. Its direction is the direction of motion.
Why it matters
Newton’s laws describe forces, but collisions happen so quickly that following every force is difficult. Momentum gives a simple account: in a closed system, total momentum stays constant. This idea grew from studying impacts and helps predict motion after a collision.
Worked example
A 2 kg trolley moves right at 3 m/s, so its momentum is 2 × 3 = 6 kg·m/s to the right. It hits a stationary 1 kg trolley and they lock together. Their total mass is 3 kg, so their shared speed is 6 ÷ 3 = 2 m/s to the right.
The common trap
A common mistake is to think that the faster object always has more momentum. That feels reasonable because speed is easy to notice, but mass matters too. Always compare mass × velocity, and include direction when objects move opposite ways.
Where it appears
Crumple zones and airbags make a crash last a little longer, so the car and passenger change momentum less violently each second. The same physics helps engineers design helmets, barriers and sports equipment. It does not remove the collision; it spreads its effect over time.
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