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i have seen newton's third law as every action has equal and opposite reactions and they act on different bodies,

my concern is is it only when both the bodies are at rest as while standing on water i m giving a force mg to water and the water should give me back mg keeping me in equilibrium,where m i going wrong.

i understand that its the normal reaction of water that should balance my weight to keep myself in equilibrium but my concern is why the normal reaction is not equal to mg,i understand it very well that as my foot presses water,the water yields,but my query is why all these things happen.

Qmechanic
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sachinrath123
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3 Answers3

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As we know from Newton's second law, F=ma. When you step on the water you are accelerating the water away from your foot, this means there is less reaction force felt by your foot because most of the force is going into displacing the water.

Karl Jessel
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while standing on water i m giving a force mg to water

Actually, this isn't happening.

You might have a reasonably strong arm and be able to push on a scale with a force of 10 pounds. Then I give you a single feather and ask you to apply 10 pounds of force to it. You won't be able to. The feather will accelerate so quickly, that it doesn't supply such a strong force/resistance to your arm.

The same thing is happening with the water. When you try to push down on it with your feet, the water accelerates away instead of pushing back (very strongly) to you.

So the third law is working and both you and the water are exerting a force on each other. The problem for you walking is that the force is less than $mg$.

BowlOfRed
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Answer lies in the fact that every matter has a property of elasticity and rigidity, then if we consider a solid road on which we walk then too we exert some deformative force but that solid is strong enough to hold it. Now when we put our feet on water then since water is not rigid enough, its molecules get displaced from their mean position. And yes every action has an equal but opposite reaction which is applicable here also considering the kind of surface you are taking, for example if you jump from a height over sea water it would behave as an equally strong road as water has from a lamina over at that place.

Aviral Gupta
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