Pressure
* Pressure is the amount of force acting on a unit surface area.
* Pressure is calculated by dividing force with area.
* Like solids, all fluids exert pressure.
* Matter that has a tendency to flow is termed as a fluid.
* Dams can break down due to the force that is caused from the pressure of the water they contain.
* Liquids exert equal pressure on all the sides of a container.
* The pressure exerted by a liquid increases with the increase in the height of the liquid column.
* The thick blanket of air that covers the earth is termed atmosphere.
* The pressure exerted by the atmosphere is called atmospheric pressure.
* The pressure inside our bodies balances the atmospheric pressure around us.
Gravitation – Thrust and Pressure
* Thrust (F) is the force acting normally on a surface.
* Pressure (p) is thrust acting per unit area.
* The thrust exerted by a body remains constants placed in any position, whereas the pressure exerted by the body changes with the change in its position.
* The mathematical equation for pressure in fluids is: P = hdg
where:
P = Pressure at the given point
h = height of the fluid
d = density of the fluid
g = acceleration due to gravity
* Pascal’s Law states that pressure applied at any point in a confined liquid is transmitted equally and undiminished to all parts of liquids and acts normally on the sides of the container.
* Application of Pascal’s law is seen in air brakes and hydraulic brakes.
* Gravitation – Archimedes’ Principle and Buoyancy
* Archimedes’ Principle states that “when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it”.
* A hydrometer uses Archimedes’ Principle to determine the density of any liquid.
* Relative density is the ratio of the density of a substance to the density of water.
* The relative density of a substance is calculated using the formula:
* Relative density = Density of Substance / Density of Water
* Applications of relative density are seen in instruments like lactometer, alcoholmeter and saccharometer.
* Buoyancy is the upward force that a fluid exerts on an object.
* Upthrust or buoyant force is the force in the upward direction, experienced by a body when it is partially or completely immersed in a fluid.
According to the principle of buoyancy:
* Objects with a density less than that of liquid float on that liquid.
* Objects with a density greater than that of liquid sink when placed in that liquid.
Pascal’s Law and Hydraulic Machines
* Pascal’s Law: when external pressure is applied at a point in a fluid contained in a vessel, it is transmitted undiminished and equally in all directions.
* A number of devices such as a hydraulic lift, hydraulic press and hydraulic brakes work according to pascal’s Law.