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

Pressure

amount of force exerted per unit area of a surface

Unit of pressure

Pascal (Pa)

1 Pascal in N/m²

1 N/m²

pressure =

force ÷ area

p =

F ÷ A

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

Relationship between pressure in a liquid and depth

as depth increases, pressure increases

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

density

pressure in a liquid column =

height x density x gravitational field strength

(liquid column) p =

hρg

Unit of density

kg/m³

Atmospheric pressure at sea level

100,000 Pa

What causes atmospheric pressure?

air particles colliding with surfaces

Relationship between altitude and atmospheric pressure

as altitude increases, atmospheric pressure decreases

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

atmospheric pressure =

height above the object x density x gravitational field strength

Upthrust

upward force exerted on a body immersed in a fluid

As water displaced increases...

upthrust increases

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

An object floats when...

its weight is equal to the upthrust

An object sinks when...

its weight is greater than the upthrust

Upthrust is equal to the...

weight of fluid displaced

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