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.
2 · Watch
Take it off screen
Where it sits
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.