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
Practical to find Young’s modulus
..m
On a graph of stress against strain what is the value of the gradient and area under the curve represent
See spcgp
How to calculate the energy per unit volume
Provide hookes law is obeyed
Energy per unit volume = 1/2 σ ε
When is can’t the young modulus be used
Beyond the limit of proportionality
What does a stress strain graph for a ductile material look like
…
What does the stress strain for a brittle material look like
…
What do stress strain graphs for, stiff strong, stiff weak, strong less stiff and weak less still look like
…
How are the molecules that make up polymeric materials arranged
Long chains of monomers
What does the stress strain graph of polyethene look like..
..
What does the stress strain graph of rubber look like
..
How do you calculate stopping distance
Thinking distance (distance traveled during drivers reaction time) + breaking distance (distance the vehicle travels after the breaks are applied)
How To prove objects all fall at the same rate
W=mg F=ma
“m” cancel
Dimension of a stable object
Low centre of mass and wide base area
What is archenemies principal
When a body is completely or partially submerged in a fluid it’s up thrust is equal to the weight of fluid it has displaced
What is the distance in calc of moment
Perpendicular distance from the pivot to line of action of the force
What is a couple
A pair of forces of equal size which act parallel to each other but in opposite directions
What do couple produce
How is it calculated
A turning force referred to as a torque
Torque of a couple (Nm) = size of the force (N) x perpendicular distance between force (m)
How to derive kentic energy equation?
….
Principal of conservation of energy
Energy cannot be created or destroyed, energy can only be transfers from one form to another but the total amount of energy in a closed system will not change
What happened in a perfectly elastic collision, what about inelastic?
In a perfectly elastic collision momentum and kinetic energy are conserved
In an inelastic some kinetic energy is converted into other forms in the collision but momentum is still conserved
Newtons 1st law
An object velocity will not change (remain at rest or move with a constant speed) unless a net force acts on it
What is an often forgot part of safety features
Crumple zones and seat belts absorb a drivers kentic energy
When can F=ma be used
Is the mass of an object is constant, then the bigger the force acting on it, the greater the acceleration (F=ma
Newton’s second law
The rate of change of momentum of an object is directly proportional to the net force which acts on the object
Newton’s third law
If object B exerts a force on object A, object A exerts and equal and opposite force on object B
(The pairs of forces are the same type eg both gravitational)
What is newtons 3rd law a consequence of
Conservation of momentum
A net force acting means a change in acceleration and therefore momentum, which must be conserved.
Dangers of safety features in cars
Seatbelts cab cause bruising
Air bags can hit passengers not wearing seatbelt with a lot of force injuring them
What does the Area under a force-time graph represent
Area under a force-time graph is impulse. Impulse imparted on an object equals the object’s change in momentum
What are the requirements of for a body to be in equilibrium according to the principle of moments.
The sum of the clockwise moments about any point has to equal the sum of the anti clockwise moments about the same point
The net force acting on it is zero
How do you determine the uncertainty in a piece of equipment
The uncertainty you can have in a piece of equipment is +/- half of one division of that instrument.
However remember if you use a bit of equipment twice the uncertainty doubles
