## Lecture 26: Elasticity - Young's Modulus

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: November 1999,   views: 35291
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)

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# Description

1. Elasticity of Materials:

A relationship between stress, strain and Young's Modulus is introduced by analogy with springs. The properties of various metals are compared, and the stress vs. strain curve is described.

2. Measuring Stress vs. Strain:

An apparatus for measuring very small elongations of a wire under stress is described. A set of measurements are made to construct the stress vs. strain curve for a copper wire. From these, both Young's modulus and the ultimate tensile strength can be calculated.

3. Spring Constant of a Wire:

At low stress values, where the stress vs. strain curve is linear, one can generate simple harmonic oscillations in the vertical direction by hanging an object from a wire.

4. Speed of Sound in Materials:

The speed of sound in a material depends on the stiffness (Young's modulus) and density of the material. The speed of sound and length of a rod determine the roundtrip time of a pressure disturbance introduced at one end. From this follows the fundamental frequency at which a rod resonates.

5. Demo with a 2 kg Block and Two Strings:

A 2 kg block is suspended from one string, and an identical string is suspended from the block. Professor Lewin pulls on the lower string. Which string will break first, the upper one or the lower one? The lower string will break first if the force is impulsive (a quick jerk) because it will elongate faster than the upper string. If we pull slowly the upper string will break first as its tension will then always exceed that of the lower string.

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1 Yash, April 18, 2010 at 6:41 a.m.:

very very thanks to Sir Lewin...regards and gratitude from India.

2 liuwen , July 6, 2010 at 5:44 a.m.:

I can not understand english

3 M.Prasad, September 21, 2010 at 6:46 a.m.:

very very thanks to Sir Lewin...regards and gratitude from India.

4 Marcelo, April 1, 2011 at 2:20 a.m.:

Thank you so much for kind innitiative, I express grattitude from Brazil.

5 M. Rao, March 2, 2012 at 6:46 a.m.:

Thank you so much, It is very intresting for me being attending in this kind of artifitial classes,
But I 'd like to have access to download some of lectures to use in my English classes for civil engineering students in our university. I am teaching in Iran
best regards
M. Rao

6 arul, August 3, 2012 at 6:37 a.m.:

thank you so much , such kind of lecture very useful for me ,
when doing work with study.

regards
prakash

7 shubham, October 15, 2013 at 7:58 p.m.:

nice one ..... please put there more material related to civil engineering .... specially strength of materials and survey ....

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