Conservation of momentum in cart collisionsStarter example
A starter example, adapted by LABKickstart from the OpenStax College Physics for AP Courses Lab Manual and shared under CC BY 4.0. It is not a classroom submission from a teacher on Benchtop.
Collide two carts on a level track twice, once so they bounce apart and once so they stick together, and test whether momentum and kinetic energy survive each kind of collision.
Materials
- Level dynamics track or a long smooth table
- Two carts of clearly different mass, one with hook-and-loop pads for the sticking run
- Lab scale to mass each cart
- Metre rule and chalk or tape to mark timing gates
- Two stopwatches, or a phone camera for video timing
Procedure
- Mass both carts and record the values. Level the track until a cart released at rest does not drift.
- Mark two timing gates a fixed distance apart on each side of the collision point.
- Elastic run: push cart A into stationary cart B so they bounce apart. Time each cart across its gates before and after the collision.
- Repeat the elastic run at least three times and keep the trials that ran straight and clean.
- Inelastic run: fit the hook-and-loop pads so the carts stick, then repeat the same measurements.
- Convert every gate time to a velocity, then compute total momentum and total kinetic energy before and after each collision.
Expected results
Total momentum comes out the same before and after in BOTH collisions, within your timing error. Kinetic energy is close to conserved in the bouncing run but drops sharply in the sticking run, which is the difference between an elastic and an inelastic collision. If momentum appears to vanish, check the track is level and that you timed the same cart you think you did.
Safety
Keep fingers clear of the collision point and do not stand carts on end where they can fall. Clear the floor at the end of the track so a cart that overshoots does not land on a foot.
Source
Adapted from OpenStax, College Physics for AP Courses Lab Manual (CC BY 4.0). Access for free at openstax.org. AP is a trademark registered by the College Board, which was not involved in and does not endorse this adaptation.
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