Collida

Meet Collida, who keeps the total momentum the same through every crash and bounce.

Momentum is mass times velocity. When two objects collide, each pushes on the other with equal and opposite force for the same time, so whatever momentum one gains, the other loses. With no outside force, the total momentum before the collision equals the total after: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. Velocities have direction, so motion to the left counts as negative.

Example: a 2 kg cart at 3 m/s hits a stationary 1 kg cart and they stick together. Total momentum is 6 kg·m/s before, so afterwards the 3 kg pair moves at 2 m/s. Kinetic energy is not conserved here: 9 J before, 6 J after, the rest going into heat and sound.

Collision investigators reconstruct car crashes using momentum. Newton's cradle, billiards and particle collisions at accelerators all obey it.

Momentum is conserved only if outside forces, like friction, are negligible during the collision. Whether kinetic energy is also conserved depends on whether the collision is elastic.