Physics - 11 Force and Pressure - 11.1 Pressure and Surfaces & 11.2 Pressure in a Liquid at Rest & 11.3 Atmospheric Pressure & 11.4 Upthrust and Flotation

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23 Terms

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Pressure

amount of force exerted per unit area of a surface

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Unit of pressure

Pascal (Pa)

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1 Pascal in N/m²

1 N/m²

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pressure =

force ÷ area

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p =

F ÷ A

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Why does a sharp knife cut better than a blunt one? [3]

- the sharp knife has a smaller area of contact
- if they exert the same force, the sharp knife exerts more pressure
- the sharp knife has a greater cutting effect

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Relationship between pressure in a liquid and depth

as depth increases, pressure increases

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Pressure in a liquid depends on the ... of the liquid

density

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pressure in a liquid column =

height x density x gravitational field strength

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(liquid column) p =

hρg

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Unit of density

kg/m³

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Atmospheric pressure at sea level

100,000 Pa

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What causes atmospheric pressure?

air particles colliding with surfaces

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Relationship between altitude and atmospheric pressure

as altitude increases, atmospheric pressure decreases

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Why does atmospheric pressure decrease as altitude increases? [5]

- the density of the atmosphere decreases with height
- so less particles collide per unit area
- so there is less pressure
- also, there is less air above an object higher up in the atmosphere
- so air above has less weight pressing down

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atmospheric pressure =

height above the object x density x gravitational field strength

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Upthrust

upward force exerted on a body immersed in a fluid

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As water displaced increases...

upthrust increases

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Why does upthrust occur? [3]

- pressure in a liquid increases with depth
- the bottom of an object experiences more pressure than the top, and therefore more force
- the resultant force of the top and bottom is the upthrust

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An object floats when...

its weight is equal to the upthrust

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An object sinks when...

its weight is greater than the upthrust

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Upthrust is equal to the...

weight of fluid displaced

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Why does density cause flotation or sinking? [4]

- upthrust = weight of fluid displaced
- if an object is denser than the fluid, its weight will be greater than the weight of fluid displaced
- so its weight will exceed the upthrust
- so it will sink