- #1
chriscarson
- 197
- 26
- Homework Statement
- Strain and change in length problem
- Relevant Equations
- Strain and change in length what is the difference ?
Strain and change in length what is the difference ?
berkeman said:What are the units of each?
So the difference is ...chriscarson said:strain no units and change of length are in m.
Well, technically the units are "1", but what units are in the numerator and denominator of the fraction that equals that "1"?chriscarson said:strain no units
No ideaOrodruin said:So the difference is ...
berkeman said:Well, technically the units are "1", but what units are in the numerator and denominator of the fraction that equals that "1"?
We don't give answers to schoolwork at the PF. When you look up mechanical strain in your textbook or on Wikipedia (see stress-strain), what units are listed? You can also use Google Images to search for stress strain curves, and a few of them will show you what I'm asking about...chriscarson said:Honestly I need answers not question
berkeman said:We don't give answers to schoolwork at the PF. When you look up mechanical strain in your textbook or on Wikipedia (see stress-strain), what units are listed? You can also use Google Images to search for stress strain curves, and a few of them will show you what I'm asking about...
What is your understanding of the difference?chriscarson said:Homework Statement:: Strain and change in length problem
Homework Equations:: Strain and change in length what is the difference ?
Strain and change in length what is the difference ?
Thank you.chriscarson said:I see, I just want the answer so I can learn nothing special but I understand you have rule.
I try to answer my question by looking for the units of these two .
Chestermiller said:What is your understanding of the difference?
berkeman said:Thank you.
And BTW, the main reason I'm asking for you to look more into this is not just because of the PF rules. Understanding the units of stress and strain is an important step in your education. The fact that the strain is "unitless" is not trivial -- there is a reason for it. It's s good step in your continued learning to figure this out, IMO.
Right, strain is the fractional change in length.chriscarson said:Because in the formula strain = change in length over length , I always thought that strain is the change in length .
Just to kick the dead horse a bit:haruspex said:Right, strain is the fractional change in length.
If a 3m bar is subjected to a longitudinal strain of 1/1000 it extends by 3mm. If we think of the bar as three 1m bars end-to-end, each is subject to the same strain, so each extends 1mm.
haruspex said:Right, strain is the fractional change in length.
If a 3m bar is subjected to a longitudinal strain of 1/1000 it extends by 3mm. If we think of the bar as three 1m bars end-to-end, each is subject to the same strain, so each extends 1mm.
thanksOrodruin said:Just to kick the dead horse a bit:
Change of length is what we call an "extensive" property, it means that if you add up change of length from each part of the bar, you get the change of length of the entire bar. This is true regardless of how you split the bar up.
Strain is what we call an "intensive" property. It is a property that does not add between different parts of the bar and (assuming equal stress and material properties) is the same everywhere on the bar regardless of the size of the part you select.
Other examples of extensive properties: mass, momentum, force
Other examples of intensive properties: density, pressure, temperature
So if I have 2 bars of lengths 1 cm and 10 cm, and I stretch them each 1 cm, the strain in the smaller bar (which is now 100% longer than its original length) is the same as the strain in the larger bar (which is now only 10 % longer than its original length)?chriscarson said:Because in the formula strain = change in length over length , I always thought that strain is the change in length .
Is it like that ...Chestermiller said:So if I have 2 bars of lengths 1 cm and 10 cm, and I stretch them each 1 cm, the strain in the smaller bar (which is now 100% longer than its original length) is the same as the strain in the larger bar (which is now only 10 % longer than its original length)?
The value of money is way too complicated to be usefully applied as a physics analogy.chriscarson said:Is it like that ...
I give 2 dollars to a man that have a 100 dollars in his pocket and
I give also a 2 dollars to another man that have 200 dollars in his pocket ,
they both benefit the same amount from me ?
They don't benefit the same amount. One guy makes 2% on his money and the other guy makes only 1% on his money.chriscarson said:Is it like that ...
I give 2 dollars to a man that have a 100 dollars in his pocket and
I give also a 2 dollars to another man that have 200 dollars in his pocket ,
they both benefit the same amount from me ?
Chestermiller said:They don't benefit the same amount. One guy makes 2% on his money and the other guy makes only 1% on his money.
2 dollars has units of dollars. It'll buy a couple hash browns at McDonalds. Strain is unitless. Like simple interest.chriscarson said:I agree compared to their sum in their pocket but as for me that the 2 dollars have the role of the strain remain the same value .I think ..
jbriggs444 said:The value of money is way too complicated to be usefully applied as a physics analogy.
I particularly like the value of a bag of grain as discussed here.
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Mmm. I seejbriggs444 said:2 dollars has units of dollars. It'll buy a couple hash browns at McDonalds. Strain is unitless. Like simple interest.
Well it’s not the same. The percentage changes are the same as strain.chriscarson said:I agree compared to their sum in their pocket but as for me that the 2 dollars have the role of the strain remain the same value .I think ..
Chestermiller said:Well it’s not the same. The percentage changes are the same as strain.
Chestermiller said:Well it’s not the same. The percentage changes are the same as strain.
Orodruin said:A buy off €2 will affect a €10 economy more than it would a €1000 economy.
A length change of 2 cm will put more strain on a 10 cm rod than it would on a 10 m rod. In terms of the material stress, strain is what matters more than change in length. Compare to the extension of two (or more) springs in series to that of each individual spring. The total extension adds up, but the strain remains constant regardless of how many strings you add.
Chestermiller said:Well it’s not the same. The percentage changes are the same as strain.
Of course.chriscarson said:So there is two things that changes when force is applied strain and length.
Chestermiller said:Of course.
Orodruin said:It should be noted that Hooke's law relates stress ##\sigma## (i.e., the applied force per area) to strain ##\epsilon## as ##\sigma = E\epsilon##, where ##E## depends on the material. Thus, from force and cross-sectional area, you can know the strain, but not the change in length (for that you also need to know the original length).
Hooke's Law is a principle in physics that states the amount of deformation (strain) of an object is directly proportional to the amount of force applied to it. This law applies to elastic materials, meaning that the object will return to its original shape once the force is removed.
Strain is a measure of how much an object deforms or changes in shape when a force is applied to it. It is typically represented as a percentage of the original length or size of the object.
Hooke's Law states that the strain of an object is directly proportional to the force applied to it. This means that as the force increases, the strain and change in length of the object will also increase.
The relationship between strain and change in length in Hooke's Law is linear. This means that as the strain increases, the change in length of the object will also increase at a constant rate.
The formula for calculating strain in Hooke's Law is strain = change in length / original length. This can also be represented as strain = F / k, where F is the force applied to the object and k is the spring constant, a measure of the stiffness of the material.