Laura sees a horse pulling a buggy. She wonders how it can accelerate if the action of the horse pulling the cart would cause an equal and opposite reaction of the cart pulling on the horse. Which explanation best answers her question?

Because action and reaction force act on different objects

Explanation:

Newton's third law of motion states that:

"When an object A exerts a force on another object B (action force), then object B exerts an equal and opposite force on object A (reaction force)".

In this situation, this law applies as follows:

1)- The action force is the pull applied by the horse on the cart

2)- The reaction force is the backward force applied by the cart on the horse

By reading 1) and 2) carefully, we notice that the two forces (action and reaction) are applied on different objects. In fact, the action force is applied on the cart, while the reaction force is applied on the horse.

This means that action and reaction force always act on different objects, never on the same object. And this explains why the horse is able to accelerate the cart: in fact, the action force applied by the horse on the cart is not balanced, and therefore the cart will experience a net acceleration.