Lab: Cart Collisions and Impulse

Put a force sensor on cart A and another on cart B, push them together with a spring bumper between them, and let them collide. Each sensor records a force-time graph during the collision. The area under each curve is the impulse delivered to that cart, since impulse equals the integral of force over time. Newton's third law says the force cart A exerts on cart B is equal and opposite to the force cart B exerts on cart A at every instant, so the two force-time traces should be mirror images, and the two areas should come out equal in magnitude but opposite in sign.

Impulse equals change in momentum, so if the impulses on the two carts are equal and opposite, their momentum changes are equal and opposite too, meaning total momentum before equals total momentum after. That is momentum conservation, read directly off the graphs instead of just assumed from theory. This lab is where the abstract bar-chart picture of momentum transfer from earlier in the unit becomes something you measure with real sensors and real numbers.

In practice the two impulse magnitudes almost never match perfectly. A gap of a percent or two is normal and expected, and it does not mean Newton's third law failed. It means the sensors have tiny timing offsets, friction or air resistance drained a little momentum from the system, or the carts were not perfectly isolated. The law holds exactly in principle; the apparatus just is not perfect.