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# Skipping of Stone on Water Surface

Its said that the stone will have maximum skips if its thrown at 20 degree from water surface..... What according to you is the reason behind spinning the stone firstly and hence throwing it????

and also the reason behind that that 20 degree angle phenomenon.. ??

Note by Yash Sharma
1 year, 11 months ago

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First of all it is obvious that the stone cannot be tilted in other direction cause that will make the stone instantly sink in Water.

Now the main thing we want is to have a good upward lift acting on stone.So Reducing the area of impact will result in a great upward force hence it cannot be horizontal and has to be some tilted.

Now we need to consider The air resistance and Take into account the Bernoulli's theorem.

Now let the angle made by stone with horizontal be $$\theta$$ and area of stone's lower surface be A roughly.

We have to adjust $$A sin\theta$$ so that air resistance is not too large and not too small because this resistance will result in an upward lift because velocity will become less and pressure will increase under the stone so we have to adjust $$Acos \theta$$ also it should be fairly big the force by air will also depend on $$A cps\theta$$ Because $$F=PA \cos\theta$$

Hence the angle should be somewhat arround 30-20$$^{\circ}$$

This is what i think. I cannot really claim that angle coz it will require practical experiments including several other factors too. This may be wrong but i wanted to convey what i thought for this. · 1 year, 11 months ago

according to me your first statement is absolutely correct.... and the third statement regarding the air resistance cannot be stated wrong ..that true...but i think air resistance wont be that major factor as compared to surface tension of the water..........i think this skipping wont take place if surface tension is absent......so what do you think about surface tension..........??? · 1 year, 11 months ago

Yes i missed it.It is also plays a major rolebut there is one thing we want the stone to remain above the water as possible so let us assume the tangent force due to ST makes an angle $$\alpha$$ with vertical as stone should be as much above water as it can be hence $$\alpha$$ will be big and hence$$T cos \alpha$$ will be less so i think it will be not that major too. $$\large\text{I think the biggest role is thrust due to water coz impact time}$$ $$\large\text{will be very less and hence impulse will be very large}$$.and and bernoulli theorem due to low velocity under it .

As we see when we strike water with great speed it feels like concrete. · 1 year, 11 months ago

and whats the main reason behind this thrust...the thrust due to buoyant force cant do that ................there would be a thrust in non-tilted position too.......and i did not understand that last Bernoulli theorem statement........ and will that be right to use Bernoulli theorem even when we are considering surface tension ..........property of non ideal fluid.??? · 1 year, 11 months ago

Hey @Yash Sharma, this is a tough one, I'll have to think about it more first! I wonder how the 20$$^\circ$$ figure is arrived at/whether it is correct. Have you tried it? Staff · 1 year, 11 months ago

yes i did try but i am still not any clear idea..........and i am not saying about why is 20 degree..........but whats the reason...it has has to be tilted in such a manner and has to be spun ..........please think and reply sir · 1 year, 11 months ago

It is an interesting question. But I think I don't have enough knowledge to comment on this. I have not studied fluid mechanics yet. :(

:) · 1 year, 11 months ago