d) If action is always equal to the reaction, explain how a horse can pull the cart

The principle of action and reaction, also known as Newton's third law of motion, states that for every action, there is an equal and opposite reaction. This might seem to suggest that a horse couldn't pull a cart, because the cart would pull back with equal force. However, this is a misunderstanding of the law.

Here's a step-by-step explanation:

1. When the horse pulls on the cart, the cart does indeed pull back with an equal and opposite force. This is the reaction. However, this doesn't mean that the forces cancel out and nothing happens.

2. The key is to consider what other forces are at play. When the horse pulls the cart, it's also pushing against the ground with its hooves.

3. The ground pushes back with an equal and opposite reaction (this is why the horse doesn't sink into the ground). This force from the ground is what allows the horse to move forward.

4. As the horse moves forward, it continues to pull the cart. The cart, in turn, moves forward because the force exerted by the horse is greater than the force of friction between the cart and the ground.

5. Therefore, despite the equal and opposite reaction from the cart, the horse is able to pull the cart because of the additional force it exerts against the ground.

In summary, the principle of action and reaction doesn't prevent movement. Instead, it's a part of how movement works.