Newton's Third Law

For every action, there is an equal and opposite reaction.

The person is pushed backward due to the reaction force.

40 N, as the ground exerts an equal force in response to the weight of the rock.

They do not cancel out because they act on different objects.

When you jump off a small boat, the boat moves backward.

Acceleration is inversely proportional to mass when force is constant.

The initial force that causes an object to move.

The force that occurs in response to an action force, equal in magnitude and opposite in direction.

The forces are equal and opposite, but they act on different objects.

When a swimmer pushes water backward, they move forward.