Force and Pressure

Pressure and Surfaces

- pressure is the force normal to a surface / area of the surface

    p = F / A

- the unit of pressure is the pascal (Pa) which is equal to 1N/m

- the force F or area A can be calculated by rearranging the pressure equation to give:

    F = p A    or    A = F / p

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Pressure in a Liquid at Rest

- the pressure in a liquid increases with increasing liquid depth

- a liquid flows until the pressure along the same horizontal level is constant 

- the greater the density of a liquid, the greater the pressure in the liquid

- the pressure p due to the column of height h of liquid of density is given by the equation:

    pressure = height x density x gravitational field strength

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Atmospheric Pressure

- air molecules collde with surfaces and create pressure on them

- atmospheric pressure decreases with higher altitude because there is less air above a given altitude than there is at a lower altitude

- the density of the atmosphere decreases with increasing altitude

- the force on a flat object due to a pressure difference is:

    the pressure difference x the area of the flat surface

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Upthrust and Flotation

- the upthrust on an object in a fluid:

    is an upward force on the object due to the fluid

    is caused by the pressure of the fluid

- the pressure at a point in a fluid depends on the density of the fluid and the depth of the fluid at that point

- an object sinks if its weight is greater than the upthrust on it when it is fully immersed

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