Suppose I apply some force on one side of hydraulic lift where area is less, and the fluid in the lift raises some heavier object on the other side where area is more. Now, work done is $\text{Force}\times \text{Displacement}$ and displacement on both side is same (incompressible liquid) but the force on one side is less, so we get more energy on other side. Then why doesn't the law of conservation of energy fail here?
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23$\begingroup$ Your statement “displacement on both sides is same” is incorrect. $\endgroup$– FarcherApr 12, 2019 at 12:22
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3$\begingroup$ do you think the levers also violate energy conservation? $\endgroup$– user8718165Apr 12, 2019 at 12:41
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3$\begingroup$ displacement means "volume", right? $\endgroup$– JEBApr 12, 2019 at 13:58
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8$\begingroup$ @JEB hits the point. Displacement here means a distance moved and not the volume displaced. $\endgroup$– JimmyBApr 12, 2019 at 14:44
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$\begingroup$ Very closely related: Why doesn't the hydraulic lift violate the law of conservation of energy? $\endgroup$– VishnuMay 18, 2020 at 10:39
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2 Answers
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Displacement in both sides is not same. If on one side of lift the area is $A_1$, and on other side it is $A_2$, and we apply a force $F_1$ on one side to distance $d_1$ then volume decreased in one side is $=A_1 d_1$
Equal amount of volume will raise in the other side.
So $$A_1 d_1=A_2 d_2$$
$A_1 \not= A_2$, so $d_1 \not=d_2$.
Actually, we need to apply the little force $F_1$ for a greater distance $d_1$.
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The displacement produced is not the same. That is why, energy is conserved.
When you apply force on one side of the opening (with smaller $A$, i.e. $A_1$), the displacement in the piston that does the work on the water is say, $x$. The displacement on the other side of the lift with $A_2$ where $A_2>A_1$, has a displacement smaller than $x$, which we'll call $y$.
What happens here is that the water absorbs energy from the piston and sends it straight to the lift on the other end with area $A_2$. The volume of water remains the same. But the displacements need not be the same.
Consider the work done $W=P\Delta V$ where $\Delta V$ is the change in volume. Since the first and the second openings are subjected to the same pressure (from the piston to the water, and from something that lifts the object in the larger opening), $\Delta V=A_1x = A_2y$.
$$y=\frac{A_1x}{A_2}$$
Since, $A_2 >A_1$, clearly, $y<x$.
