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Just a little background before I present my question - I am a Psychology graduate looking to move into Neuroscience so I'm going through some Physics to learn more about this topic as it is relevant for some Neuroscience courses. As such I lack some background knowledge of Physics and have last studied it at GCSE about 5 years ago, so forgive me if my question is a little basic!

I'm going through a section on Newton's Laws at the moment and my textbook gives an example of a Newton Pair, identifying the gravitational pull of a person on the Earth and the contact force of the person on the Earth as the Newton Pair. Right underneath it says that to help identify Newton Pairs, you should remember they have to be the same type, e.g. both gravitational forces.

In the above example, it does not seem that the forces are the same type, but I have seen other similar examples, so feel I am missing something. I'd really appreciate it if somebody could explain this to me a little better, including why those two are a Newton Pair if they are different types of force and what I am missing in this example.

Thanks!

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  • $\begingroup$ it would help if you could cite some of the aforementioned similar examples. I have an idea where this question is going... $\endgroup$ Nov 3, 2013 at 17:26
  • $\begingroup$ Sorry, don't have any, they were mainly from pages when I was looking this up, but they essentially seemed to suggest that two different types of force formed the pair. $\endgroup$
    – AAM
    Nov 3, 2013 at 20:36

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It sounds like an error on the part of your textbook.

The third-law pair for the Earth's gravitational force on you is your gravitational force on the Earth.

The third-law pair for your contact force with the ground is the ground's contact force, AKA normal force, on you.

The contact force on the ground can't be a third-law pair with gravitational force. Try jumping off a roof top, for example. Then there would still be a gravitational force on you, but no contact forces (for a short while).

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  • $\begingroup$ Thanks, I was wondering if it was, or whether there was just something I wasn't understanding! It has another example in the book which carefully matches up 4 different pairs, which looks right to me, which made me even more confused. I think I know what its getting at now though, but I think it may be worded badly/I'm reading it badly, or a combination of the two. Thanks for the help! $\endgroup$
    – AAM
    Nov 3, 2013 at 20:39
  • $\begingroup$ There are some more interesting concepts here, normal force/contact force from the ground is a result of believe it or not, electromagnetic forces. $\endgroup$ Nov 4, 2013 at 3:01

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