Question

NCERT CLASS IX - Science Textbook
Chapter 9 - Force and Laws of Motion

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

Answer 1

       The horse exerts force on the ground.  It pushes the ground backwards.  Then the ground exerts force on the horse in the forward direction.  So it moves forward relative to the ground.  The ground being massive, moves very little backwards.

     The horse is attached to the cart.  The attachments and hence the cart is pulled by the horse in the forward direction.  The cart moves forward due to the force exerted by the horse on the cart.

   The cart exerts equal and opposite force on the horse.  So  the horse does not move with respect to the cart.  Both move together as they exert forces equal and opposite.

   The force that is causing both cart and horse is the reaction force from the ground on the horse.

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.