Recoil and momentum conservation
They push apart, yet the total momentum stays zero.
Two carts rest on a frictionless track with a compressed spring between them. When released, the carts move off in opposite directions: no external force acts on the system, so the total momentum stays zero, and the lighter cart moves faster. All of the spring's elastic potential energy becomes the carts' kinetic energy. Rockets and jet aircraft rely on the same recoil principle.
Cart velocities vs. time
At release the carts gain opposite velocities; speed is inversely proportional to mass.
Think about it
Cart A has 2 times the mass of cart B. After they separate, which one is faster, and by how much?
Cart B moves 2 times as fast as cart A. The total momentum is zero: m₁v₁ = m₂v₂, so speed is inversely proportional to mass; the two carts' momenta are equal in magnitude and opposite in direction.
Model assumptionsFrictionless track · Massless spring, all elastic energy becomes kinetic energy · Released after 1 s at rest
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.