Chapter 6 Flashcards
What are compressive and tensile forces
Compressive forces are any pair of forces that shorten the length if an object
Tensile firces are any pair that extend them
Explain the graph of forces against extension for tensile forces (or compressive) acting ina spring
1) for a while, the force is proportional to the extension, under hooked law. This is until the PORPEOTIONAILKTY LINKT
2) past the properijanlity limit, the extension is no longer proportional. However until it reached the ELASTIC LIMIT, if the firces is removed, the spring will still deform elasticsllt and return back to Orginal length
3) once it reaches elastic limit and goes past this, any energy transferred here will be used to permantley alter arrangement of atoms and thus PLASTICALLG DEFORM the spring , energy is not retuned. Plastic deformation
Hooked law thus only applies to the propetijality limit
Hooked law
Here force is directly proportional to the extension, thus f= is, where k is the constant in the equation
K is known as the SPRING CONSTANT or stiffness in Nm-1
Where is K on force ere soon graoh and what does till you
Basics,ky force / ex = k so the GRADIENT is the spring vinstant in the proportional region
Springs with higher spring constant are more stiff, and thus need more force for same extension = this would look like a higher gradient on the graph !
What is proper word definition fir k
Why does this not hold forever and why can’t we jus say you need this much force extaclt to stretch it by like 10p0,
Spring constant is a measure of how much firce is needed ti extenedthe object by 1 m
In reality This doesn’t hold (as we see past proprtnisly limit), the l changes making it harder to stretch.
What happens to spring constant calculations with multiple springs in
- series
- parallel
1) in series
- in series it is EASIER TO PULL FOR THE SAME DISTANCE . This is because they are connected, so the same force is transmitted across BOTH springs, so basically they both extend a bit, making extension big
So the formula similar to resitsntce
1/ktot = 1/k1 + 1/k2
2) parallel
This is harder!
Makes sense, to pull one by 1m , gonna be harder to pull two by one meter , instead you’re gonna pull both by half if you apply the same force as before
Same as resistance in series, the total Ktot = k1 + k2
How to investigate spring constant in PAG
So abdiall Lamp everything does took cg
1) set up a clamp stand and a clamp , set with a SET SQUARE to ensure this is perpendicular
- ALSO SECURE TO BENCH USING CLAMPS for safety, and clamp a METER RULER TOO to measure
2) then attach a spring , and measure
3) now can add mass and record extension
Make sure you record at eye level to reduce systematic error
Repeat
Yh again when using hooked law what ssumptuon yiu make
Thst the object HASNT REVAHED THE PRORPTINALUTH LIMIT YET, FIR WHICH THE FORMUSLR STOPS WORKIGN SS SPRING FISNTSJT DHANGED
How is accuracy imorived in this experiment?
Using a set square and measuring t from eye level
What to remember for all this about compression too
That it follows all the same rule, elastic limits eberhtijfn
What happens to energy when extending snd when is it recovered or lsot
When you extend something until elastic linit, the energy is stired in the object, which can be returned when force is removed
However when you extended something past, not all energy can be received as some was used ti structurak,y alter the object in plastic deformation
Work done by spire gmoving?
This is known as elastic port is energy
And for this energy fan be reviverer
Work done = f x X
So area
Area = 1/2 fx
F=kx
So area = 1/2kx2
So what is wrong with the spring constants and why is there a better thing out there
Spring constant can be dependent on
- length, the longer it is, the easier it is to pull, so K is lower with length longer
- cross sectional area, if you make this bigger, then harder to pull, k is bigger for higher area
Basically k is clearly DHALE DEPENDENT, but you don’t want that, want something like rrsitivity which is independent ofmshape and size
What is tensile stress
Stress = the force / cross sectional area
So Nm-1
What is tensile strain
Strains = a ratio of new length to original length, extend / length
Has no units, is just a quantity
Extension is actual extension not new Lenten gtw
So what is young modulus and why is it good
Young modulus e= stress/ strain = FL/AX
What you’ve done here is basically if are and length is 1, then they have no effect and you still have a value for property of materials, different material still hold one
The units are NM-2,mor pa(?)
Bridge question for tensile bs compressive
Again the top of bridge is hsoterning si co prestige, bottom stretching so tensile
Explain stress strain graph for an alloy, and what is an alloy
An alloy is a material made from a combimantionif metals for benifits which can be different sized atoms ,
All we’ve done with graph is applied some constant and rescaled growth nothing changed
1) similar to first graph, stress is proportional to strain until proportionality limit= this means object obeys hooked law until here
2) from here to elastic limit, object still deforms elasticallg
3) past here it will deform plasticsllt = energy not recoverable
-so from here itsplasticsllt deforming whilst extending until it reaches a point , this is the first yield point Y1
- at this point, the atoms in the alloy (due to being different sizes) UNLOCK, and rapid acceleration is caused,
Thin about it, if accelerates quickly (like untiring a knot), then less force is needed to extend it, thus it DIPS in the force whilst extension still increases.
However like a knot in string, it LOCKS AGAIN , and at this point it is the second yield point y2
Note that this is tyOical for alloymliek steel bpmaybe nti be in Akku alloys
- from yield 2man increase in force is required to extend and plasticsllt keep on deforming . Normal and curved because not proportional
- this keeps happening until it reaches the UTS , ultimate tensile strength . This is a point where the material can take MAXIMUM FIRCE AND STRESS before it breaks
Thus a higher UTS means a stricter material
Past the UTS the process of breaking happens , here it may extend a bit more with less force in a process called necking (simialr ti like stretching blu tack it stretches a lot and thins for a bit), until you reach the BREAKING POINT, past this the Karteikarten break
Summary again and UTS ?
Ultimate tensile force, higher value for this means a material is more stronger
So does First Part the same, prosperously, elastic limit and then plasticsllt deforms
L- reaches a point where atoms unlock= y1, then rapid acceleration occurs making it easier to extend with less force, so it dips to y2, where atoms lock again
Then plastic continues, extension increases with FIRCE but curved because not proeptijal
Happens u til you reach UTS which si amd stress it can take befire breaking
Past this it may neck, where the thing thins and less force required for a bit more extension so dip
However finally itnrevshed breskig. Lost snd obkct snaps I half
How to calculate hound modulus from a stress strain graph
Again the gradient until proportionality limit
Why do we compare materials young modulus
A material with higher young modulus is stiffer than a lower one , which is dependent solely on material and not like thin or bit p
So thin copper and thick will have same young modulus! Useful in cimrdtructuon
