Question

A person applies 3 N of force to a shopping cart. The cart's momentum changes by 6 kgm/s. Under what conditions does the person's momentum change by the same amount?

a) All conditions, this is the physical law

b) If there is much fricition between the person and the floor

c) If the person is much heavier than the cart

d) If all forces on the person are negligible except for that applied by the cart.

Answer 1

Answer: (d)

If the momentum of the person changes by the same amount, it can only happen if there is no external force. When external force on a system is zero, momentum of the system remains conserved. Here the system comprises of the cart and the person. The force applied by the person on the cart is the action and the reaction of it is the equal and opposite force that the cart applies on the person. Only if there is no other force acting on the person, the change in momentum will be the same for both (in opposite directions). To imagine this, consider a person standing on ice and pushing the cart. We can imagine easily that the person will be pushed back as he/she pushes the cart forward.