Question

1. If action is always equal to the
reaction, explain how a horse can
pull a cart.​

Answer 1

Action and reaction though equal and opposite to each other act on different bodies hence do not cancel each other. That's why a horse can pull a cart on Earth but a horse cannot pull a cart in space due to the same I.e. Newton's third law of motion (theoretically).

Answer 2

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When the horse walks in the forward direction (with the cart attached to it), it exerts a force in the backward direction on the Earth. An equal force in the opposite direction (forward direction) is applied on the horse by the Earth. This force moves the horse and the cart forward.

The velocity at which the horse can move by applying a force on the earth depends on the mass of the horse (and the cart attached to it). The heavier the cart, the slower the motion of the horse (for a given amount of force applied by the horse on the Earth). If the cart is too heavy, the force exerted by the horse on the Earth will be insufficient to even overcome the force of inertia. In this case, the horse will not be able to pull the cart.