A cart of mass 0.400 kg moves with a speed of 1.2 m/s toward a secondary cart of mass 0.300 kg that is initially at rest. When the carts collide they stick together. What speed do the carts move with after the collision

0.686 m/s

Explanation:

The principle of conservation says the sum of momenta before collision is equal to the sum of momenta after collision in an isolated system. Here, we assume the system is not acted upon by any external force.

The initial momentum = momentum of 1 st cart + momentum of 2nd cart

                                     = 0.400 × 1.2 + 0.300 × 0 = 0.48 kg m/s

The final momentum = (0.400 + 0.300) × v   (since they stick together)

0.700 v = 0.48

v = 0.686 m/s

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.