P11: Materials Flashcards
Density, Hooke's law, Stress and Strain, Young Modulus, Stress-Strain and Force-Extension graphs, Brittle materials
Define ‘Stiffness’
The force needed to extend an object by 1m.
Depends on its material, length and shape.
This is also equal to the object’s spring constant.
What is an object’s ‘Young Modulus’?
A measure of its stiffness.
Young Modulus formula is…?
E = tensile stress/ tensile strain = (F)(L)/(A)(x)
Where x = the extension.
Brittle
Means doesn’t deform much when a force is applied and quickly snaps.
Plastic materials
Are those which deform permanently (plastically) after being stretched; the atoms within it have changed position permanently.
Elastic materials
Deform when stretched, but return back to their original shape once the force is removed; its atoms return to their equilibrium positions.
For a metal this happens as long as Hooke’s Law is obeyed.
What is an object’s limit of proportionality?
The point when an object stops obeying Hooke’s Law.
What is an object’s elastic limit?
The point when the material begins behaving plastically.
What is an object’s yield point?
The point from which the material starts to stretch without any extra load.
What does the area under a stress-strain graph represent?
The energy stored in the material per unit volume.
Which 2 quantities have base unit kg m^2 s^-2?
Work done and the moment of a couple.
What is meant by an object’s centre of mass?
The point through which the applied force will cause no moment
What is a couple?
A pair of forces of equal size acting parallel to each other but in opposite directions.
What are the units for spring constant?
Newtons per metre
