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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 13

Lesson 1: Stress, strain, and modulus of elasticity

# Stress & strain

To measure the elasticity of any material we need to define two quantities. Stress and strain. In this video let's explore what these are and why we define them? Created by Mahesh Shenoy.

## Want to join the conversation?

• can we say that everything in this universe has elasticity up to some point?
• At , if we assume the longer string to be made of 5 strings, doesn't that also mean that we will be stretching 5 strings at once(like the 5 times the thickness )?
Shouldn't that increase the restoring force.(5 times)
• when there is one string it will stretch for 0.1cm(say) and for 5 strings it would be (0.1+0.1+0.1+0.1+0.1)cm {5 times for 5 strings}.So the ratio of stretching would remain same. We know for f/a is proportional to d(l)/l so if d(l)/l and a(cross sectional area or thickness ) would remain same then f would remain same. Similarly when there is an increased thickness and d(l)/l remained same f increases to make ratio f/a constant.
• When we stretch a wire, not only does its length change but, the cross sectional area changes too.
So while calculating the stress on a string, which cross sectional area should we consider?
The earlier one or the changed one?
• The change in area is generally considered negligible (unless the question states otherwise) for small forces. Hence we take the original cross-sectional area for calculations. In case the question requires you to consider the changes in area, we would have to sum up (integrate) each individual stress (usually as a function of time) for every formed length and corresponding cross-sectional area.
• can we say that " stress is the cause of strain" ?
• Yes,
strain (however microscopic) causes stress. i.e stress is dependent on the strain.
Strain is directly proportional to deformation = (more the deformation = more the change in dimension)
Think about it logically, what is strain?
= The restorative force developed per unit area,
the key term here is restorative force = molecules/atoms/ions are trying to restore themselves to their equilibrium stable positions, why would they need to do that if they are not displaced?
This is also why the x-axis represents stress (dependent variable) and the y-axis strain (independent variable) in stress vs strain graphs and the moduli of elasticity.
Hope this helps,
Onward!
• is it possible to stretch wavelengths of light?Does light have elasticity?
• Light behaves in accordance to something called the Dual Nature Theory, which basically states that light sometimes behaves like a wave and other times as a particle (photon).

Now, elasticity is applicable only to physical objects, not for waves. Stretching or compression of the wavelength of a wave can be caused due to the movement of it from and to materials of optical densities but this doesn't imply elasticity of light.
• but there is no way to stretch a stone how does it has elasticity?
• elasticity of rocks can be observed by compressing them at microscopic level
• this video is very informative.
• You should write that in the Tips and Thanks" Section.
This is the question section
(1 vote)
• What if the object does not always have a consistent cross-sectional area, like a tire. How do you determine the area needed to calculate stress? Do you take the average?
• how can the strength of a beam relate to the thickness
(1 vote)
• I have two cases:
1) As an object is stretched its area reduces so shouldn't the stress developed increment (like integrating the small increments for a small decrease in area == total tensile stress)?

2)similarly, as an object is compressed its area increases so shouldn't the stress developed decrease (like integrating the small decrements in stress for a small increase in area == total compressive stress)?

If the above reasonings are correct(more or less), then, we can say Tensile stress is lesser than Compressive stress for a given load (respective directions)
Is this why most materials can withstand greater compressive load than tensile load?
What about metals, why are they almost the same (Ycompression ≈ Ytensile)?
I might be way off, but it's just a thought :)
(1 vote)