Properties of Matter

Moving fluid - predicting distances travelled by the pistons

Physics Narrative for 11-14 Supporting Physics Teaching

Moving fluid: shows how hydraulic systems work in a fundamental way

Now for a thought experiment. Suppose you try moving liquid from the left hand side of the hydraulic system to the right hand side and then adjust the pistons so that they sit on top of the fluid.

The smaller the cross sectional area of the input cylinder, the less liquid needs to be removed before the piston can drop. But this liquid has to go somewhere, and the only place to go, in a sealed system, is to the other output cylinder. Given that the cross-sectional area of this cylinder is larger, the piston will rise less for each cubic metre of liquid fed into it.

These two cylinders constitute a hydraulic system, where a large movement on one side (the input side) creates a smaller movement on the other (the output side).

This confirms the point made earlier that the relative area of the cylinders determines the relative up and down movement of the two pistons. If the second cylinder is bigger the travel of the piston in that cylinder is smaller. As we have just shown, this is due to the fact that no liquid is lost as it moves from one cylinder to the other.

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