Forces and Matter
● Hooke’s Law
○ Load is proportional to extension of a stretched spring
○ Δx (extension) = stretched length - original length
● F = kΔx
○ F = force
○ k = spring constant
○ Δx = extension
● Spring constant
○ Force/ extension
○ S.I unit is N/m
○ Parallel springs → k = k1 + k2
○ Joined springs → 1/k = 1/k1 + 1/k2
● Force is directly proportional to extension
● Once spring loses elasticity and deforms, it doesn’t go back to original form
● Force applied per unit area is known as stress
● The extent of stretching/ compression produced as a material responds to stress is known as
strain
● Extension → F= kx
● Compression → F = -kx (just a notation, not actually applied)
● Types of deformation
○ Elastic deformation
■ When the stress is removed the material returns to the dimension it had before
the load was applied. Deformation is reversible
○ Plastic deformation
■ When large stress is applied to a material, when removed, the material does not
spring back to the previous dimension. Permanent and irreversible deformation
● Minimal value of stress which produces plastic deformation is known as elastic limit of the
material
Pressure
● Pressure = Force/ area
● S.I unit is N/m2 or Pascal(Pa)
● 1 N/m2 = 1 pascal
● Solids
○ Pressure = Force/Area
○ Less area → More pressure
● Remember to always turn cm to m when finding pressure
● Liquids
○ P = hdg
■ h = height(metres)
■ d = density(kg/m3)
■ g = gravity(10N/kg on earth)
○ Atmospheric pressure is the pressure exerted by air/ atmosphere
○ Pressure caused by liquid = hdg
○ Pressure in a liquid = pressure caused by liquid + Atmospheric pressure
● Relations
○ Density is directly proportional to pressure
○ Height is directly proportional to pressure
○ Gravity is directly proportional to pressure
Manometer
● An instrument used to measure the pressure between 2 gases or liquids.
Its a U-shaped tube holding a small amount of liquid
● When both ends are open, the level of the liquid on either side is equal.
● If one end is connected to a gas supply, the gas pushes down on the
liquid and forces it round the bend
● The levels are now unequal, showing that there is a pressure difference
● The greater the pressure, the further it is pushed
● When atmospheric pressure is greater than the gas supply, then it is
known as suction
● Suction = atmospheric pressure - difference in height
● Gas supply = atmospheric pressure + difference in height
Barometer
● An instrument used to measure atmospheric pressure
● Consists of glass tube at least 80cm in length
● The tube is filled with mercury(Hg) and then carefully inverted
into a trough containing mercury
● Once the tube is safely inverted, the level of the Hg in the
glass tube drops
● Length of the Hg column is measured from the surface of the
Hg in the trough is about 76cm
● The space above the Hg column is a vacuum
● As atmospheric pressure decreases, length BD decreases
Both point J and K exert the same pressure
