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1. Force exerted on ground by a man
Question:
How much force a walking man is exerting on ground
a) equal to his weight
b) more than weight
c) less than weight
d) a force not related to weight?
I thought that it should be equal to weight, because according to newton's third law of motion there must be a force acting on ground as a reaction to the Normal force of ground on the man. Apparently, that's wrong as the mark scheme gives n answer of (b).

I have no idea how B can be the answer. Any help/hints would be appreciated.
2. Re: Force exerted on ground by a man
Surely if he were standing still it would be equal to his weight, but if he is moving at some rate downwards because of gravity, he carries more force as there is velocity involved?

I have no idea, really. That's just a guess. I didn't even take GCSE physics.
3. Re: Force exerted on ground by a man
My reason for assuming its because of movement is that it seems analogous to a car; a car moving at 100mph has more force than one stationary, right?

This makes my brain hurt.

I'm going back to contract law.
4. Re: Force exerted on ground by a man
(Original post by Zishi)
Question:

I thought that it should be equal to weight, because according to newton's third law of motion there must be a force acting on ground as a reaction to the Normal force of ground on the man. Apparently, that's wrong as the mark scheme gives n answer of (b).

I have no idea how B can be the answer. Any help/hints would be appreciated.
Go and jump on some scales and you'll find the answer.
5. Re: Force exerted on ground by a man
(Original post by James82)
Go and jump on some scales and you'll find the answer.
Sorry, I don't have access of scales right now. Could you please answer it qualitatively?
6. Re: Force exerted on ground by a man
When you are walking you are pushing on the earth.

So the force will be greater than the weight.
7. Re: Force exerted on ground by a man
(Original post by Zishi)
Sorry, I don't have access of scales right now. Could you please answer it qualitatively?
When the man is standing still the man exerts a force on the ground (his weight) and the ground exerts an equal force on the man (ground reaction force).

When the man wants to walk he has to exert a greater force onto the ground, so by using his muscles he exerts a downward and backwards force (thus increasing the force exerted onto the ground to more than his weight, which answers the original question). This extra force is opposed by both an increased ground reaction force and the friction between the ground and the man's foot, thus causing the man to move forwards. By the way, this also causes the earth to spin backwards in relation to the man, otherwise Newton's 3rd law would fail, but due to the immense mass of the earth relative to the man it is obviously not noticeable!
Last edited by James82; 06-07-2012 at 11:22.
8. Re: Force exerted on ground by a man
Also unless he is standing at the poles you need to take into account the effect due to circular motion and a centripetal force.
9. Re: Force exerted on ground by a man
If the man is standing still, a) would be true.
If the man is walking, he is forcefully pushing against the ground. Therefore he applies a force which is equal to his weight + the extra force making b) true for that scenario.

Don't get too worked up about the grounds resistance or any of Newton's law because they aren't applicable to this question .
10. Re: Force exerted on ground by a man
(Original post by Yellowmonkeyman)
If the man is standing still, a) would be true.
If the man is walking, he is forcefully pushing against the ground. Therefore he applies a force which is equal to his weight + the extra force making b) true for that scenario.

Don't get too worked up about the grounds resistance or any of Newton's law because they aren't applicable to this question .
Don't forget though that after forcefully pushing, the man would then be going up as well so the force oscillates.
11. Re: Force exerted on ground by a man
(Original post by Augmented hippo)
Also unless he is standing at the poles you need to take into account the effect due to circular motion and a centripetal force.
12. Re: Force exerted on ground by a man
(Original post by TheGrinningSkull)
Don't forget though that after forcefully pushing, the man would then be going up as well so the force oscillates.
Absolutely, therefore the assumption is that I'm referring to the maximum force exerted by the man
13. Re: Force exerted on ground by a man
(Original post by Keckers)
You can imagine it as the earth is spinning so you are being slightly flung of the earth therefore you exert a smaller force on the ground than your weight.
14. Re: Force exerted on ground by a man
Alright. Am I correct to assume that force on ground is at angle of more than 90 degrees from the ground so to provide the forward force(which would be horizontal component of force on the ground) to make the man move?
15. Re: Force exerted on ground by a man
(Original post by Augmented hippo)
You can imagine it as the earth is spinning so you are being slightly flung of the earth therefore you exert a smaller force on the ground than your weight.
Do you understand the use and application of the word 'negligible'?
16. Re: Force exerted on ground by a man
It will be slightly more as he needs to exert force to sustain motion against friction. When standing still it is equal to weight.
17. Re: Force exerted on ground by a man
(Original post by Keckers)
Do you understand the use and application of the word 'negligible'?
Brutal, it comes out to a measurable 0.03* mass between the equator and poles if you assume the earth to be a perfect sphere of constant density.

oh wait...
Last edited by XiaoXiao1; 06-07-2012 at 23:35.
18. Re: Force exerted on ground by a man
(Original post by Keckers)
Do you understand the use and application of the word 'negligible'?
perhaps I'm just in a good mood but this actually made me laugh.
19. Re: Force exerted on ground by a man
(Original post by boba)
perhaps I'm just in a good mood but this actually made me laugh.
Perhaps it's because I'm a comic genius

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