physics - forces, density & pressure

Centre of Gravity

The point at which the weight of an object is considered to act. Objects are more stable with a wider base (Lower CoG)

Moment

Turning effect of a force
Moment = Fd

Couple

A pair of forces that act to produce rotation only. Forces in a couple are equal in magnitude and opposite in direction

Torque

Moment caused by a couple of forces.
Torque = Fd(perp distance between both forces)

Principle of Moments

For a system to be in equilibrium, the sum of clockwise moments must equal the anticlockwise moments about the same point

Equilibrium

When all the forces are balanced, meaning there is no resultant force or torque

Density

The mass per unit volume of an object
p=m/V

Pressure

Pressure is the force per unit area
P=F/A

Hydrostatic Pressure

Pressure at any given point in a fluid, exerted by the weight of fluid above that point

Derivation of deltaP=rho g deltah

1. Weight of fluid above point W=mg

2. Density = m/V so m=DensityV

3. W=rho V g

4. V=Ah so W = rho Ahg

5. P=F/A so P=rho g h

6. deltaP = rho g deltah

Add atmospheric pressure for total pressure

Upthrust

Force that pushes an object in a fluid upwards, caused by difference in hydrostatic pressure between the top and bottom of the object

Archimedes Principle

An object submerged in a fluid at equilibrium has an upwards buoyancy force equal to the weight of fluid it displaces

Buoyancy Force

F=rho g V