Chapter 4 Forces

centre of mass

imaginary point where the entire weight of an object appears to act

drag

frictional force that opposes the direction of movement, through a fluid. proportional to speed squared

moment of a force

turning effect of the force about some axis

principle of moments

body in rotational equilibrium the sum of anticlockwise moments about any point is equal to the sum of clockwise moments about the same point

couple

pair of forces that are equal and parallel along different lines

torque

the moment of a couple

one of the forces x perp distance

density

mass per unit volume

pressure

normal force exerted per unit cross sectional area

why do fluids exert pressure

constant bombardment on surfaces by molecules

p proportionate to height

water pressure increases with depth. denser liquids exert greater pressure

buoyant force/upthrust

force at bottom - force at top to give Axpg

equal to the weight of the fluid displaced

Archimedes principle

upthrust exerted on an object fully/partially submerged in a fluid is equal to the weight of fluid displaced

sink or float

object will sink if upthrust less than weight / more dense than fluid

weight of floating object = weight of fluid displaced