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Could someone explain to me why this 'perpetual motion machine' would not work? The only explanation i've thought of is it would be impossible to suspend the water in a way where the balls could still travel through it...

idea

(the idea is the balls on one side are lifted (in water) and on the other side they are dropped, it cycles due to constantly differing buoyancies)

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At the point where the balls enter the water from the bottom they will experience "negative buoyancy" that exactly cancels out any work done by the balls floating up in the water - to enter the water column you have to do work against the pressure caused by the full column of water.

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    $\begingroup$ From your answers, I am guessing you have an engineering background as well.......:) $\endgroup$
    – user154420
    Jun 15 '17 at 16:00
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    $\begingroup$ @Countto10 physics by training, engineer by profession. $\endgroup$
    – Floris
    Jun 15 '17 at 16:01
  • $\begingroup$ My next question is wouldn't the buoyancy of a ball leaving the liquid cancel out the negative buoyancy of one entering at the same time? $\endgroup$ Jun 17 '17 at 10:14
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    $\begingroup$ @LucasMartin no - the pressure at the bottom is higher than at the top so more work has to be done to enter the liquid. $\endgroup$
    – Floris
    Jun 17 '17 at 13:29
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You can imagine some iris- like door that opens when the ball at the bottom tries to go up. But the machine will not work because the water preasure will push the ball down. It will not allow the ball to enter from the bottom. Remember that at this point the ball is not fully inside and there is a net force down due to the preassure differential between water and air.

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  • $\begingroup$ I wrote this answer as floris posted his. It is essentially the same. $\endgroup$
    – user126422
    Jun 15 '17 at 15:43
  • $\begingroup$ i think your answer uses more approachable vocabulary. $\endgroup$ Jun 15 '17 at 16:30
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As Arthur implies below, my answer is a (wrong) argument for perpetual motion, rather than against it.

Imbalance of weight. The more spheres you use to achieve greater bouyancy, the more you are left with on the left hand side, so a balance will eventually be arrived at, no matter what number of spheres are involved.

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    $\begingroup$ No, that's not it. Bouyancy on the right and weight on the left is exactly what would make this go round. $\endgroup$
    – Arthur
    Jun 15 '17 at 17:23
  • $\begingroup$ @Arthur. Thank you, you are completely correct and I need to go back to basic principles, or else think before I answer. $\endgroup$
    – user154420
    Jun 15 '17 at 22:07

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