Conservation of Momentum
Billiards — 2D elastic collisions. Builds on Momentum & Impulse.
An astronaut floating in space (no friction, no air) throws a heavy tool bag away from them.
What happens to the astronaut?
They drift backward! Total momentum starts at zero and MUST stay zero (conservation of momentum). If the bag gains momentum to the right (+), the astronaut must gain equal momentum to the left (−). This is literally how rockets work — throw mass backward, you go forward.
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Watch two objects interact — their total momentum bar never changes, no matter how violently they collide.
m₁v₁ + m₂v₂ = m₁v₁' + m₂v₂'
Momentum conservation is MORE fundamental than Newton's laws — it works in quantum mechanics and relativity too. It's a consequence of the universe being the same everywhere (translational symmetry).
When the cue ball hits a stationary ball head-on, it stops dead — all its momentum transfers to the target. That's perfect momentum conservation.
A bullet (small m, big v) goes forward. The gun (big m, small v) kicks backward. Total momentum = 0 before and after. Same principle, same physics.
Two skaters push off each other on ice. They slide apart with momenta that are equal and opposite. The lighter skater moves faster.
“Conservation of momentum is one of the deepest principles in physics. Emmy Noether proved it comes from a symmetry of space itself — the universe has no preferred location.”
Collision Lab
Set up collisions between objects of different masses and velocities. Verify that total momentum is always conserved.
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Set up a head-on collision between equal masses, one moving and one stationary. The moving one stops and the stationary one takes off — perfect momentum transfer!
Total momentum is conserved in EVERY collision — elastic, inelastic, or explosive. It's the most reliable bookkeeping rule in physics.