Unit 5, lesson 25. About 15 minutes.

Collisions

How does a snooker ball know exactly how to pass on its speed?

Play

Getting the experiment ready…

Explain

Things bump and share their push.

Some bounce, and some stick.

When things collide, they pass momentum to each other. Add up the momentum before and after, and the total is the same.

In a bouncy collision, things spring apart, like snooker balls.

In a sticky collision, they join together, like two lumps of clay. Try both.

Conservation of momentum: with no outside force, total momentum stays constant.

In an elastic collision, kinetic energy is kept too. Equal masses swap velocities. In a sticky collision, kinetic energy is lost to heat and sound.

Worked example: 2 kg at 6 m/s hits and sticks to 4 kg at rest. So 12 = 6 × v, and v = 2 m/s.

Total momentum before
Bump!
Same total after

Want the full story?

Real collisions are between these extremes, and a perfectly elastic one almost never happens with everyday objects. The sim leaves out friction and spin, which matter in snooker. Newton's cradle shows the same idea, with one ball in and one ball out.

The Formula

total momentum before = total momentum after

m_1 v_1 = (m_1 + m_2)\,v

$m_1$: the mass of the moving cart, measured in kilograms (kg)
$v_1$: its speed before the crash, measured in metres per second (m/s)
$m_2$: the mass of the still cart, measured in kilograms (kg)
$v$: the speed of both, stuck together, measured in metres per second (m/s)

Try the numbers

Moving cart: 2 kgStill cart: 2 kgSpeed: 5 m/s

Speed if they stick: 2.5 m/s

Try it at home

Ask a grown-up to help

You need two coins, a smooth table.

You need: two coins, a smooth table.

Roll one ball at another. See it move!

Put a coin on a smooth table.
Flick a second coin straight at it.
Watch which one moves after they hit.

What's happening: the moving coin passes on its momentum, so it nearly stops while the other coin moves away.

Coins are a choking hazard, so keep them away from small children's mouths.

Physics is life

Your body

Your head and a ball collide.

Your bones and joints absorb bumps as you walk and run.

Daily life

Snooker balls bounce off each other.

Crash tests measure collisions, so that engineers can design safer cars.

Nature

Big animals butt heads.

Rams butting heads have thick skulls to survive the collision.

Space

Asteroids crash on planets.

Collisions made craters on the Moon, and may have formed the Moon itself.

Quick check

Question 1 of 3

What stays the same in a collision?

Words to know

Collision: When two objects crash or bump together.

Collision: When two objects crash or bump together.
Momentum: Mass multiplied by velocity. It tells you how hard something is to stop.
Elastic collision: A bouncy collision, where no movement energy is lost.

Next up

Why seatbelts and helmets save lives