|
Virtually all materials
have some give to them. When pulled on, they stretch. For
many materials this give is not apparent to the naked eye but
it is there. Some materials, natural rubbers and synthetic
elastomers have a great deal of give. We use them every day. Rubber
bands, tires, shoes, an wide assortment of plastic products and many
metal products as well.
When objects are
stretched (or compressed) and then return to their initial
dimensions when the forces that produced the change are removed
removed, the objects are said to be elastic in
behavior within that range of loading.
When the objects no
longer return to their initial state, suffering permanent
deformation, they have exceeded their elastic limit.
Materials that we
typically do not consider elastic are usually very soft and
malleable approaching to the point of approaching a
fluid-like consistency. Examples include clay, some plastics, and
very soft metals such as gold or lead. These materials begin
have very low elastic limits and tend to stretch significantly
before actually reaching the limits of their tensile strength and
failing.
Other materials not
generally thought of as elastic are very hard, usually brittle ,
often with highly oriented crystalline structures; glass, titanium,
cast iron. These materials are the opposite of the previous one.
They tend to have very high tensile strengths and are normally quite
rigid. But unlike the clay they require very large loads to deform
in any way. The problem with these materials is that when they reach
their elastic limit they have also reached the limit of their
tensile strength and immediately fail, often catastrophically.
This is most apparent in
aircraft and race car design. Carbon fiber composites have extremely
high strength to mass ratios and can be made into very light , very
strong, and very rigid structures. They are often five times
stronger, or five times lighter that the metal components they have
replaced.
The drawback to these
products is that when they fail, they fail in a catastrophic manner.
Shattering or breaking rather than bending or stretching.
In this chapter there are
example and problems using springs and modeling their behaviour
based on Hooke's Law. F=kx. All of the examples assume that the
springs are operatiog wll within their elastic limits and that they
are not being exposed to excessive cycles or conditions. As springs
are used they loose their elasticity with number of cycles they are
put through. The also lose their linearity (if they were liner
intially) with use.
USE ONE OF THESE TO ANSWER YOUR SPRING PROBLEMS
The Physics of Springs and Spring-Like Systems Project - ELASTIC
and SPRING SIMULATIONS
http://www.glenbrook.k12.il.us/gbssci/phys/projects/yep/springs.html
NTNU JAVA - GOOD!
http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=7
Spring Oscillator Applet
http://lectureonline.cl.msu.edu/~mmp/kap13/cd361a.htm
SPRING SIM -Hooke's Law
Game and Simulation Programming
http://cs.millersville.edu/~webster/cs406gamesimulation/cs406gamesimulation.html
Hooke's Law simulation
http://www.mhhe.com/physsci/physical/jones/ol14-1.htm
The educational encyclopedia, physics: mechanics java applets
http://users.pandora.be/educypedia/education/physicsjavalabomechanics.htm
SPRING SIM -Hooke's Law
|
Fullerene
Graphite
'intercalated'
with another element
