Physics
Hydraulic machines work under the Principle of :

Joules Law
Pascal's Law
Floatation Law
Newton's Law

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Physics
On a clean glass plate a drop of water spreads to form a thin layer whereas a drop of mercury remains almost spherical because:

cohesion of water is greater than its adhesion with glass.
cohesion of mercury is greater than its adhesion with glass.
mercury is a metal.
density of mercury is greater than that of water.

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Physics
Stationary wave is formed by

Two waves of same frequency travelling in the opposite direction
A transverse wave superposing a longitudinal wave
Two waves of same frequency travelling in the same direction
Two waves of the same speed superposing

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