# Can you help me with a basic question on Newtons 3rd law - I have a question

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#1
I am only asking about the "stationary on earth part" Here is the question that I am referring to:

I know that there would be equal and opposite contact forces (with the man pushing down on the box, the box pushing up on the man) and I know there would also be equal opposite gravitational weight forces (earth pulling on man/box, box/man pulling up on earth). These would be the Newtons 3rd Law pairs.

The mark scheme just said this:
"Weight/downwards force (on person) -1 mark
There is an upward force on person (from the floor) OR reaction/contact force - 1 mark
No resultant force OR person not accelerating -1 mark
Therefore the reaction/contact force from the floor upwards = Weight - 1 mark"

I don't get why you only need to talk about the contact force upwards and the weight/gravitational force downwards. Why don't you also need to talk about the opposite gravitational force upwards and the equal contact force downwards? I know these forces would be there, as newtons third law says they are in pairs, and my textbook also says that, but why don't we need to mention them here? Why doesn't the man feel the equal opposite weight force acting upwards in his feet? Why does the man only feel the contact force upwards in his feet?

I don't get that. MY teacher said it was something to do with the question only wanting what the man would feel in his feet, but I still don't get why you only need to mention the contact force up and the gravitational force down. (and not the gravitational force up and the contact force down)

Thanks a lot!
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4 years ago
#2
I don't think you need to mention it because the whole equal opposite reaction thing which means they must exist regardless I don't think you'd lose aany marks for saying they exist just aslong as you mentioned those specific ones again I'm not certain but that's the conclusion I came to. (Just because I re read your post the man does feel those things they just don't need mentioning)
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#3
I drew it on paper. I don't get why the mark scheme only mentions (and my teacher also said) it only applies to the two forces I drew a rectangle around in blue pen. Why doesn't the man feel the other two forces in his feet?

Attachment 615402

I just cannot understand why the man wouldn't feel in his feet the force of the man pulling the earth up (the upwards gravitational force)
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