A-Level Physics: Materials: Overview

archimedes’ principle

states that the upthrust experienced by an object is equal to the weight of the fluid it displaces

upthrust=

weight of fluid displaced

how can the weight of displaced fluid be calculated?

by finding the product of the density and volume of displaced fluid, multiplied by gravitational field strength

formula for upthrust

pVg

what is the volume of the displaced fluid for a fully submerged object?

volume of the object

what is viscous drag force?

resistive force experienced by an object moving in a fluid

what is the term for the resistive force experienced by an object moving in fluid?

viscous drag force

what can be used to calculate viscous drag force exerted on an object?

Stoke’s law

what are the conditions for Stoke’s law?

the object must be small and spherical, and moves slowly with laminar flow

what is the equation for stoke’s law?

6 x pi x viscosity x radius x terminal velocity

what is laminar flow?

where particles flow by following smooth paths with little to no mixing between layers of path

what is turbulent flow?

where particles mix between layers of path and form separate currents

what is viscosity?

a measure of how resistant a fluid is to deformation, for example caused by an object moving through it

what is a fluid’s viscosity determined by?

the internal frictional forces occurring between adjacent layers of fluid

what variable affects viscosity?

temperature

what is the effect of temperature on the viscosity of most liquids?

as temperature increases, the viscosity of a liquid decreases

what is the effect of temperature on the viscosity of a gas?

as temperature increases, the viscosity increases

why are the effects of temperature on viscosity of liquids and gasses different?

frictional forces between layers are formed by different means

what does Hooke’s law state?

extension is directly proportional to the force applied

what are the conditions of Hooke’s law?

external conditions, such as temperature, remain constant

what is the equation for Hooke’s law?

stiffness x change in length (extension)

what is stress?

force applied per unit cross sectional area

what is the formula for stress?

force/area

what is the unit of stress?

Pa or Nm^-2

what is strain?

change in length over the original length

what is the formula for strain?

extension/length

what is the relationship between stress and strain?

for a material which obeys Hooke’s law, stress is proportional to strain

what can be said about the relationship between stress and strain?

the value of stress/strain is constant

what is the value of stress/strain equal to?

Young Modulus

what is young modulus?

a value which describes the stiffness of a material

what is the derived formula for Young Modulus?

(force x length)/(area x extension)

what is the limit of proportionality?

point after which Hooke’s law is no longer obeyed

what is the elastic limit?

point just after the limit of proportionality

what happens if you increase the force applied beyond the elastic limit?

the material will deform plastically

what is the yield point?

point at which material begins to stretch without an increase on the load

what is elastic deformation?

where a material returns to its original shape once the applied force is removed

why does elastic deformation occur?

as all the work done is stored as elastic strain energy

what is plastic deformation?

where a materials shape is changed permanently

why does plastic deformation occur?

as work is done to move atoms apart, so energy is not only stored as elastic strain energy, but also dissipated as heat

what do force-compression graphs show?

how the compression of an object varies with the force applied to it

why do force-extention and force-compression graphs often look very similar?

solids often behave similarly when tensile and compressive forces are applied

what is the key difference between force-extention and force-compression graphs?

beyond the elastic limit, compressed solids will suddenly change shape and break as opposed to extending plastically

what is a stress-strain graph?

graph describing the behaviour of a material

how does a stress-strain graph differ from a force-extension graph?

they describe behaviour of a material rather than specific object

what can the shape of a stress-strain graph show?

whether a material is ductile, brittle or plastic

what is a ductile material?

a material that can undergo a large amount of plastic deformation before fracturing

what is a brittle material?

a material undergoes little to no plastic deformation before fracturing

what is a plastic material?

a material which experiences a large amount of extension as the load is increased

what is the breaking stress?

the value of stress at which the material will break apart

what does the breaking stress depend on?

dependant on the conditions of the material for example the temperature

what is elastic strain energy?

energy stored when work is done to deform a material

how can the elastic strain energy be found from a graph?

area under a force-extension graph

why can elastic strain energy not be calculated by formula W=F#s?

as the force is variable not constant

what formula can be used to calculate elastic strain energy?

(force x extension)/2

what is shown on a force - extension graph once a material is stretched past its elastic limit?

the graph showing loading and unloading lines will not return to the origin

what can be said about the loading and unloading lines on the force/extension graph? Why?

the loading and unloading lines will be parallel as the material's stiffness remains constant

how can the work done to permanently deform the material be found from a force-extension graph?

from the area between the loading and unloading line