Question

If action and reaction forces are always equal and opposite, explain how a cart drawn by a horse moves forward

Answer 1

The horse applies a force F to the Earth, and the Earth (as a reaction) applies a force -F to the Horse.

As a result, the Horse moves in the direction of -F and since we are applying a torque to the Earth, it either decreases or increases its rotation speed by a corresponding value.

Ok, with the cart, building on the previous example.

The original force F can make the horse move at a velocity 2v.

Now, the Horse applies a force F to the Earth and starts to move at velocity 2v, Then when the slack in any ropes is taken up, a force 0.5Fis applied to the cart by the horse and a force -0.5F is applied by the cart to the Horse.

Since the Horse is outputting a force F and half of it is going to the cart, only 0.5F can now be applied to the Earth, and the Horse & cart only moves at velocity v, since the Earth only applies an equal but opposite force of -0.5F.

Answer 2

$\bigstar\green{\underline{\sf{\purple{Answer}}}}\bigstar$

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.