Question

why dont equal and opposite forces, like a person pushing a boat and the boat person the person back, cancel each other out?
because they are not strong enough to affect motion.
because they act on different objects.
because they only occur in water.
because one is stronger than the other.

Explanation:

Newton's third law states that for every action, there is an equal and opposite reaction. However, these forces act on different objects. In the case of a person pushing a boat and the boat pushing the person back, the force from the person acts on the boat, and the reaction force acts on the person. Forces cancel each other out (resulting in no net motion) only when they act on the same object. Here, since the forces act on different objects (the person and the boat), they don't cancel each other in terms of affecting the motion of a single object. For example, the force on the boat can make the boat move (if it overcomes other resistive forces like water resistance), and the force on the person can affect the person's motion (e.g., if the person is on a friction - less surface, they would move in the opposite direction).

Answer:

Because they act on different objects.