04 Materials Flashcards
Accuracy definition
How close you are to the true value
Sensitivity definition
How small a division a piece of equipment can measure to
Precision
How similar repeated values are
What causes upthrust
Caused by the displacement of water
When an object is submerged in a fluid, what determines the upthrust experienced?
The upthrust of a submerged object is equal to the weight of the fluid that it displaces
How to calculate upthrust (equation)
Density of fluid x volume in water x acceleration due to gravity (9.81)
What determines whether an object floats or sinks?
The balance between the weight and the upthrust of the object. If the weight exceeds the upthrust, the object will sink
Why will a uniform object, with a density greater than that of the liquid, it is submerged in, always sink
- the upthrust of the object will be equal to the weight of the fluid displaced
- therefore, the maximum upthrust will be equal to the density of the fluid x volume of object x gravitational field strength
- the weight of the object is equal to the density of the object x volume of object x gravitational field strength
- if the objects density is greater, the weight will always be greater than the upthrust and the object will sink
What shaped objects does stokes’ law apply to?
Stokes’ law only applies to small spherical objects
What type of flow is required for Stokes’ law to apply
Laminar flow
What does Stokes’ Law allow you to calculate
The viscous drag force that a small spherical object experiences when falling at low speeds through a viscous fluid with Laminar flow
Stokes’ Law equation
F=6πηrv
η= the viscosity of the fluid
r= the radius of the sphere
v = the velocity of the sphere
F= frictional force acting on the interface between the fluid and the particle.
What’s elastic deformation
An object has undergone elastic deformation if it returns to its original shape once the deforming forces are removed
What is plastic deformation
An object has undergone plastic deformation if it no longer returns to its original shape once the deforming forces are removed. It will have permanent deformation
Express Hooke’s Law in words
The extension of an elastic object is directly proportional to the force that is applied to it, up to its limit of proportionality
What is the limit of proportionality
The point beyond which the force and the extension will no longer be directly proportional to each other - Hooke’s Law is no longer obeyed
What is the elastic limit
The point beyond which the object will no longer elastically deform, and will instead deform plastically
State the defining equation for Hooke’s law
F = kΔx
F= force applied (N)
K= stiffness constant
Δx = extension (m)
What is mechanical stress
The force experienced by an object per unit area
State the equation for stress
Stress = F / A
F = the force applied
A = the cross-sectional area
What is the unit for stress
Nm^-2
State the equation for strain
ΔL / L
ΔL = the change in length
L = original length
What is the unit for strain
Strain is a unitless quantity because it is the ratio of 2 lengths
What does the Young Modulus of a material tell you
A material’s Young Modulus is a measure of how much force is required for a given extension, regardless of the objects dimensions
What equation is used to calculate a materials Young Modulus
Young Modulus = stress / strain
Which is the gradient of the stress strain graph with strain on the x axis and stress on the y axis
What unit is used for a materials Young Modulus
Nm^-2
What is breaking stress
Breaking stress is the maximum stress an object can withstand before fracturing
What is the yield point
The point beyond which the object will experience a large extension without substantial increase in the force applied
The point at which the amount of stress the material is at causes the wire to begin to plastically the deform
What type of energy is stored in an object that has been stretched
Elastic potential energy
State 2 equations used to calculate the energy stored in a spring
E = 1/2 F Δx
E = 1/2 k Δx^2
x = extension
What does the gradient and area represent on a force-extension graph
- the gradient of the linear region represents the elastic constant (k)
- the area is equal to the elastic potential energy stored in the spring
Equation to find out the volume an object is submerged
Volume submerged = upthrust / (density x g)
What does the area under the line of a of a force/extension graph show and which (force or extension) is on the x axis and which is on the y axis
Y axis = force
X axis = Extension
The area under the line shows the work done to stretch
what does the gradient of a force extension graph represent and define it
The spring constant - a measure of the stiffness of the spring
Define upthrust
the upward force that a liquid or gas exerts on a body floating in it.
Derive the equation of a falling object ( Weight = upthrust + drag) to get it equaling terminal velocity
V = 2r^2 g (density of object - density of fluid) / 9η
During laminar flow, in what way do the fluid layers move
Parallel to each other at different speeds
Required conditions for stokes law to apply
Small
Spherical
Slower speed ( so it doesn’t go into turbulent flow)
Laminar flowing liquid
Why does the spherical object for stokes law have to be small
If it’s big, the friction between the fluid and the wall will be too big, disrupting, the viscosity
Why can you not use work done = f x d when trying to work out the work done in a spring
Because the force is not constant
Brittle definition
Hard but liable to break easily
Ductile definition
Able to be deformed without losing toughness
Polymeric definition
A material that is composed of many monomers
Describe the shape of a stress strain graph of a brittle object
It will reach its elastic limit quickly and can’t go past it for very long
Describe the shape of a stress, strain graph of a ductile object
It will be able to be past its elastic limit for longer
What does the area under a stress strain line represent?
Work per unit of volume
How do you work out the strength of an object from a stress strain graph?
The highest stress on the line
what is the effect of temperature on viscosity and why
Viscosity will decrease with increased temperature because as particles move more quickly, they interact for shorter time (shorter interactions) reducing internal friction or stress and therefore decreasing viscosity.
Where on a graph would you put the limit of proportionality
When it just starts to curve
Where on a graph would you put the elastic limit
Just beyond the limit of proportionality
Where on a graph would you put the yield point
After this point, it gets much flatter
Compare a ductile material to a brittle material
The brittle material has only a linear-elastic region before rupture failure while the ductile materials has a linear-elastic region followed by a region in which plasticity engages.
What is breaking stress
The maximum stress a material can stand before it breaks is called the breaking stress or ultimate tensile stress
If the student had used the larger sphere, the equation for viscosity would not have produced the correct answer.
Explain why.
• With the large sphere the speed will be greater so Stokes’ law won’t apply
• The flow is turbulent or not laminar
How does viscosity change when temperature in a gas increases
viscosity of gases increases with an increase in temperature.
Yield point definition
Stress at which it stops behaving elastically
Why might it be difficult to use stokes law on a bubble
Only for small solid spheres so there will be less drag due to the bubbles having a non-stationary surface
