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Physics/Newton's third law


"Hello Sir
I have one question to ask related to physics at its basic level. I am 10th standard student and last year my science textbook made me familiar with newton's third law formally.
There was a question in my textbook - If every action has equal and opposite reaction then explain how a horse can pull a cart?

I found an answer to this question from an unknown source - when horse pulls a cart then cart also pulls it with same force but horse also pushes the ground which in turn pushes the horse ahead and hence both horse and cart move.

But, I find this answer very much confusing.....
1. How can a cart apply a force if it doesn't have any source of       energy?
2. There it was written that cart applies the same force in opposite direction as compared to the horse, then cart should not move ever as whatever force one will apply on cart, the same will be applied by cart.But this is not so. Why?
3.let us suppose horse applied force +f and cart applied -f according to the newton's third law, then horse also pushed the the ground with same magnitude of force and therefore ground pushed the horse also. So now total force applied by horse on cart is +2f then why don't cart exerts -2f force on horse as per the law. But this is not the case, why?
4. If it is incorrect explanation of the mentioned phenomenon then what should be the correct answer?

1)  The cart has mass.  The tension forces in the rope are electrostatic in nature, causing a slight polarization of the molecules in the rope and the cart.  Accelerating the cart (applying a force to it), causes this polarization, the horse is still the energy source.
2)  The cart does not apply a force to itself, it applies a force to the horse.  In the absence of another force, the cart accelerates.
3)  No, the second sentence is wrong.  The force of the ground is applied to the horse (and it is greater than just f, that force from the ground has to accelerate both the horse and the cart).  It is not applied to the cart.  This also answers 4).

This problem I was reluctant to answer, because it reads like homework.  Don't directly ask me homework problems that you have not made a serious attempt to solve without telling me what you did and just asking for a hint.  As this problem is conceptual in nature, I answered it.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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