<< Chapter < Page Chapter >> Page >

Check Your Understanding Explain why the concepts of Young’s modulus and shear modulus do not apply to fluids.

Fluids have different mechanical properties than those of solids; fluids flow.

Got questions? Get instant answers now!

Summary

  • External forces on an object (or medium) cause its deformation, which is a change in its size and shape. The strength of the forces that cause deformation is expressed by stress, which in SI units is measured in the unit of pressure (pascal). The extent of deformation under stress is expressed by strain, which is dimensionless.
  • For a small stress, the relation between stress and strain is linear. The elastic modulus is the proportionality constant in this linear relation.
  • Tensile (or compressive) strain is the response of an object or medium to tensile (or compressive) stress. Here, the elastic modulus is called Young’s modulus. Tensile (or compressive) stress causes elongation (or shortening) of the object or medium and is due to an external forces acting along only one direction perpendicular to the cross-section.
  • Bulk strain is the response of an object or medium to bulk stress. Here, the elastic modulus is called the bulk modulus. Bulk stress causes a change in the volume of the object or medium and is caused by forces acting on the body from all directions, perpendicular to its surface. Compressibility of an object or medium is the reciprocal of its bulk modulus.
  • Shear strain is the deformation of an object or medium under shear stress. The shear modulus is the elastic modulus in this case. Shear stress is caused by forces acting along the object’s two parallel surfaces.

Conceptual questions

Note: Unless stated otherwise, the weights of the wires, rods, and other elements are assumed to be negligible. Elastic moduli of selected materials are given in [link] .

Why can a squirrel jump from a tree branch to the ground and run away undamaged, while a human could break a bone in such a fall?

In contact with the ground, stress in squirrel’s limbs is smaller than stress in human’s limbs.

Got questions? Get instant answers now!

When a glass bottle full of vinegar warms up, both the vinegar and the glass expand, but the vinegar expands significantly more with temperature than does the glass. The bottle will break if it is filled up to its very tight cap. Explain why and how a pocket of air above the vinegar prevents the bottle from breaking.

Got questions? Get instant answers now!

A thin wire strung between two nails in the wall is used to support a large picture. Is the wire likely to snap if it is strung tightly or if it is strung so that it sags considerably?

tightly

Got questions? Get instant answers now!

Review the relationship between stress and strain. Can you find any similarities between the two quantities?

Got questions? Get instant answers now!

What type of stress are you applying when you press on the ends of a wooden rod? When you pull on its ends?

compressive; tensile

Got questions? Get instant answers now!

Can compressive stress be applied to a rubber band?

Got questions? Get instant answers now!

Can Young’s modulus have a negative value? What about the bulk modulus?

no

Got questions? Get instant answers now!

If a hypothetical material has a negative bulk modulus, what happens when you squeeze a piece of it?

Got questions? Get instant answers now!

Discuss how you might measure the bulk modulus of a liquid.

Got questions? Get instant answers now!

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'University physics volume 1' conversation and receive update notifications?

Ask