If action is equal to reaction, explain how a horse can pull a cart?
Answers
Hey Pretty Stranger!
The horse exerts some force on the cart, and the cart exerts an equal & opposite force on the horse. So, at first glance it seems that the action & reaction forces being equal & opposite cancel out & hence the cart wouldn't move. But it should be noted that it is only the force on the cart that determines whether the cart will move or not, and that the force exerted by the cart on the horse affects the horse alone. Thus, if the horse is able to apply enough force to overcome the frictional force present, the cart will move. So, to make the cart move, the horse bends forward & pushes the ground & it's feet. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.
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.