How would you calculate the total momentum for two carts that are about to collide?

Answers will vary. To calculate the total momentum for two carts that are about to collide, calculate the momentum for each cart and combine the two. Note that if carts are moving in opposite directions, the velocity of one of the carts is negative.

Answers will vary. The total momentum of two carts after a collision will equal the momentum of the two carts before the collision (if the net external force is zero).