Momentum and Its Conservation
Momentum is mass times velocity, and it points the way the object moves. With no outside force, the total momentum of a group of objects is the same after a collision or explosion as before.
What a learner can do afterwards
- Calculates momentum in kg m/s and gives its direction
- Works out an unknown velocity after two trolleys collide and stay joined
- Applies the same reasoning to an explosion, where two objects start at rest and move apart
1 · Read
Momentum measures how much motion an object carries, which matters when you brake a bike or catch a ball. You find it by multiplying mass by velocity. Its unit is kilogram metres per second, written kg m/s. Because velocity has a direction, momentum points that way too.
A 2 kg trolley rolls at 3 m/s. Multiply 2 by 3 to get 6 kg m/s in the direction it rolls. A slow lorry can share the same momentum as a fast tennis ball when the numbers work out. Big momentum means hard to stop.
When trolleys collide with no outside force pushing in, the total momentum stays the same, because the action-reaction pairs between them cancel. Add every momentum before, and the total after must match. If two trolleys stick together, one joined lump carries it all, so the same momentum shared by more mass means a slower speed. Explosions run the same rule backward from rest. The total starts at zero and stays at zero, so the parts move apart in opposite directions with momentums that cancel. Pick one direction as positive and keep the signs straight.
Explosions run the same rule backward. Two trolleys start at rest with zero total momentum, then push apart in opposite directions. Pick one direction as positive and keep the signs straight, because opposite momentums cancel to zero.
Mass times velocity gives momentum, and the total stays the same through crashes and explosions.
More to read
8.1 Linear Momentum and Force
Momentum measures how much motion an object carries. Multiply its mass by its velocity to get it, in kilogram metres per second. Because velocity has a direction, momentum has one too, so always say which way things move.
Big momentum means hard to stop. A slow lorry and a fast tennis ball can share the same momentum if the numbers work out. Forces change momentum, and bigger forces change it faster.
8.3 Conservation of Momentum
When objects collide with no outside forces pushing in, the total momentum stays put. Add up every momentum before the crash, do the same after, and the two totals match. This works for crashes, explosions, and rockets alike.
Explosions are collisions played backward. Two objects start at rest with zero total momentum, then fly apart in opposite directions. Their momentums cancel exactly, keeping the total at zero.
8.5 Inelastic Collisions in One Dimension
Some crashes stick the objects together, like two trolleys coupling on impact. Momentum is still conserved, but now one joined lump carries it all. The same total momentum shared by more mass means a slower speed afterward.
To find the unknown velocity, total the momentums first with careful signs. Set that equal to the combined mass times the final velocity, then solve. Some kinetic energy is lost as heat and sound, but the momentum math stays exact.
Explore more
2 · Watch
Take it off screen
Where it sits
Optional connections
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.