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You are correct in suggesting that the cart pulls back on the horse. The best way to picture this is to consider a horse attached to a cart by a 100m rope coiled up. The horse is allowed to run away as fast as it can. When the rope runs out, the horse will most likely get pulled almost to a stop in what will be a very dramatic surprise for the horse! But the cart will start moving. However much momentum the horse had when it hits the end of the rope will be transferred to the cart. So the cart does indeed pull on the horse, but in doing so receives some (probably most) of the horse's momentum. That's how Newtonian Physics works. Newton's Laws basically evaluate to the following statement: Momentum is preserved in any system that receives a zero outside force, and the change in momentum of any system is the change in the product of the mass and (directional) velocity of the system. If we can assume that the horse and the cart represent a closed system, then if the horse comes down to 10% of its original speed, the velocity of the cart will be the horse's decrease in velocity multiplied by the ratio of the horse's mass to the cart's mass. So if the cart has 2x the mass of the horse, it will be moving at 45% of the horse's speed and in the same direction because the horse pulled on the rope, the rope pulled on the cart, accelerating it, the cart pulled back on the rope, and the rope pulled back on the horse, stopping it. Now if the horse burns some of the grass it ate and uses it to generate some energy which it then transferred to the earth using its legs, we would need to include the Earth in the momentum system. Since momentum has to be preserved, however much momentum the horse imparts to the Earth, the Earth will push back. The horse will appear to be moving relative to the Earth, but keep in mind that the Earth has been pushed in the opposite direction. Now bring the rope's length down to essentially zero and consider this chain of events being repeated rapidly. The reason why your original point doesn't work is because in some cases the system consists only of the horse and the cart, and sometimes it consists of the horse and the Earth. First the horse does a momentum exchange with the Earth, then it does a momentum exchange with the cart. This process repeats over and over again and the (horse + cart) system is moving relative to the earth