Key ideas1 of 3
Build a signed momentum total
Conservation of momentum applies to a closed system with no resultant external force. Take right as positive and use p = mv for each object before adding. A 3 kg trolley at +4 m/s has momentum +12 kg m/s. A 2 kg trolley at −1 m/s has momentum −2 kg m/s. The total is +10 kg m/s, rather than 14 kg m/s.
