All Difficult Stuff Module 2,3 Flashcards
What is hooke’s law
F=kx
x is extension is m
F is force in newtons
K is the force constant (for a wire) / the spring constant (for a spring) (units Nm^-1)
Does the value of k change depending on the wether the force applied is compressive and tensile
No
Draw a graph of a typical force extension graph for a spring
What can be said at the separate points on the graph
The first part of the graph (up to point P) shows Hooke’s law being obeyed - there’s a straight-line relationship between force and extension.
When the force becomes great enough, the graph starts to curve.
Metals generally obey Hooke’s law up to the limit of proportionality, P.
The point marked E on the graph is called the elastic limit.
If you exceed the elastic limit, the material will be permanently stretched.
When all the force is removed, the material will be longer than at the start.
What can be said about the “k” of springs in parallel and seres
If a force is applied to more than one spring (or wire), you can combine the force constants of the individual objects to find the overall force constant of the system.
Series 1/K = 1/k(1) + 1/K(2)
Parallel K=K(1) + K(2)
What is the type of deformation before the elastic limit is reached? What about after? Z
A material will show elastic deformation up to its elastic limit, and plastic deformation beyond it.
If a deformation is elastic, the material returns to its original shape once the forces are removed.
How does extension work on an atomic level, for elastic deformation?
When the material is put under tension, the atoms of the material are pulled apart from one another.
Atoms can move slightly relative to their equilibrium positions, without changing position in the material.
Once the load is removed, the atoms return to their equilibrium distance apart.
How does extension work on an atomic level for inelastic (plastic) deformation
Some atoms in the material move position relative to one another.
When the load is removed, the atoms don’t return to their original positions.
How can hooked law be investigated
CGP pg 45
How can the tensile stress on an object be calculated?
σ=F/A
Force applied
Cross sectional area
Stress has the units Nm^-2
How can tensile stain be calculated?
ε=x/l
Strain has no units
Extension over original length
How do you calculate compressive stress and strain?
The same equations only you tend to think of tensile forces as positive while compressive forces are negative
What does a graph of stress against strain look like for a material
See CGP page 46
What is represented by point B (name) on the graph of stress against strain in the CGP
Eventually the stress becomes so great that atoms separate completely, and the material breaks.
This is shown by point B on the graph. The stress at which this occurs is called the breaking stress.
What is the UTS
The point marked UTS on the graph is called the uitimate tensile strength. This is the maximum stress that the material can withstand before breaking.
How to calculate work done on graph of force against extension
On a graph of force against extension, the work done is given by the area under the graph.
Equation to calculate elastic potential energy?
What if force isn’t constant
E=1/2 Fx
E=1/2 Kx^2 (assuming hookes law is being obeyed)
the force on the material isn’t constant. It rises from zero up to force F.
To calculate the work done, use the average force between zero and F,
Does the material still gain elastic potential energy after it is stretched beyond the elastic limit
If the material is stretched beyond the elastic limit, some work is done separating atoms.
This will not be stored as elastic potential energy and so isn’t released when the force is removed.
What is the young modulus equation and units
Young modulus units is Nm^-2
Equation = Fl/Ax
How is the Young’s modulus used ?
Measure of stiffness of a material
