Materials Flashcards
What is Hooke’s law?
Change in extension is proportional to change in force.
ΔF = kΔx
Where F is force, k is the spring constant and x is extension
What does Hooke’s law apply to?
When you apply a pair of opposite forces.
E.g compression and extension
What is the limit of proportionality?
The point in which Hooke’s law doesn’t work anymore
What is the elastic limit?
The point in which the material would have been permanently streched
What is a property of rubber?
It only obeys Hooke’s law for very small extensions
What is elastic deformation?
When a material is put under tension, atoms are pulled apart but are still in their equilibrium position in the material so it can return to it’s original shape
What is plastic deformation?
When some atoms in the material have moved relative to each other so they cannot go back to their original position
What is tensile strength?
When you stretch a material
What is compressive strength?
When you compress a material
What is the formula for compressive and tensile stress?
Stress = F/A
Stress = Force / Area
What is the formula for tensile and compressive strain?
Strain = Δx/x
Strain = Δextension/original length
What is the breaking stress?
When the stress starts to pull atoms apart from one another
How can you calculate the energy stored in a stretched material?
Workdone = 1/2 * Force * ΔExtesion
and since ΔF = k*Δx
Δworkdone = 1/2 * k * (Δx)^2
What is Young’s modulus?
The constant found by dividing the stress by the strain
Young’s Modulus = Stress/ Strain
It’s a measure of the stiffness of a material aka
How much force is needed for a given extension
How would you experiment to find the Young’s modulus of a material?
Use a thin and long test wire
Find cross-sectional area
Clamp wire to bench
Measure a fixed distance
Add weight and measure extension
Calculate the stress and strain
How would you use a stress-strain graph to find Young’s modulus and energy?
Plot stress on the x and Strain on the y and then find the gradient
Energy is the area under the curve
What is the yield point?
The material starts to stress without extra load, the stress sin which a large amount of plastic deformation occurs
What is density?
The mass per unit of volume
Mass/Volume
What is pressure?
The amount of force applied per unit area
Pressure = force / area
What is fluid pressure?
Pressure = Depth (h) * desnsity * gravitaional field strengh
What is upthrust?
An upwards force that a fluid exerts on a material that is partially or fully submerged in fluid
What is Archimedes’s principle?
Upthrust = Weight of the fluid displaced
What is viscous drag?
Friction between an object and fluid. Calculated using Stoke’s law
What is Stoke’s law?
Viscous drag = 6πnrv
n is the viscosity of the fluid
r is the radius of the ball
v is the velocity of the ball
