What is your explanation to convince the horse to pull?
A farmer urges his horse to pull a wagon. The horse refuses, saying to try would be futile for it would contradict Newton's 3rd law. The horse concludes that she can't exert a greater force on the wagon than the wagon exerts on her, and therefore won't be able to accelerate the wagon.
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The farmer should apply his whip, at which point, all logic would fly out the window! But seriously, folks...
The horse is correct to a point. Newton's 3rd law correctly states for every force, there is an equal and opposite reaction force. The horse pulls against the wagon. The wagon has mass, which is subject to the law of gravity, so it exerts a force against the Earth of so many pounds of force pulling back against the horse. As long as the horse's pulling force does not exceed the force of the wagon's weight pulling back against the horse, the forces are balanced, and no acceleration occurs, as the horse stated.
The farmer should, at this point, tell the horse that horses have been used since antiquity because it is a well known fact that horses are able to exert a much greater force than the wagon's weight is able to counter. The farmer should encourage the horse to pull harder, that at the point the horse's force exceeds the wagon's weight, the wagon will indeed begin to move. So the horse's logic is flawed, after a certain point, the horse's force will exceed that of the wagon, and acceleration will begin.
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