# What is elasticity?

1. Jun 11, 2015

### Tam Le

I read somewhere that elasticity is the measure of the tendency of an object to resist deformation (similar to how inertia resists motion). I also read elsewhere that elasticity is the measure of how well an object returns to its shape/form after removing the applied force.

Which definition is correct? Are they two sides of the same coin: An object with a high elasticity value would be tough to deform, but it would also be able to return to its original form/shape more or less accurately. An example or picture illustrating the concept would be preferable.

2. Jun 11, 2015

### hackYou

Those two definitions do not contradict each other so both of them are correct.

An object tries to resist deformation while it is put under stress (one or more forces are acting - trying to deform it) and that same object will try to return to it's original form/shape(given that no plastic deformations occur) after the aforementioned forces stop acting on it.

3. Jun 11, 2015

### Tam Le

Thank you for your explanation (just don't hackMe ).

4. Jun 12, 2015

### Noctisdark

Elasticity isn't a measure if how well an object can return to it's initial form, if it doesn't return to it's initial shape it isn't elastic, it's plastic in that case, elasticity is the ability to return to initial state, all materials exhibit elastic behavior, then at a certain exerted force they become plastic

Last edited: Jun 12, 2015
5. Jun 12, 2015

### A.T.

6. Jun 12, 2015

### hackYou

7. Jun 12, 2015

### nasu

The confusion may come from the fact that Young's modulus (Y or E) is also called "modulus of elasticity" or "elastic modulus".
However a large value of E means that the material is stiffer and not "more elastic".

If you are looking for some kind of a measure of how elastic a body is, you may look at how much can be deformed before becoming non-elastic. This may be described as "yield point" or "yeld strength" (if the stress is measured).

8. Jun 12, 2015

### Tam Le

That makes sense. Thank you A.T.

9. Jun 12, 2015

### Merlin3189

Just wanted to query this point. Do brittle materials exhibit plasticity? I just had the notion that brittle materials are elastic up to the point of failure.
Just looking at WikiP I see they say that, brittle materials exhibit little or no plastic deformation.

And as far as the foregoing discussion is concerned, I had not come across the idea that terms like elastic, plastic and brittle were defined. I have always taken them as mere qualitative descriptions of the properties of materials.

10. Jun 12, 2015

### Noctisdark

Yes all material can become plastic, but that plastic zone is very tiny for brittle one, meaning once.it become plastic, it can break easily by adding force

11. Jun 13, 2015

### jerromyjon

This is the first I have heard of plastic in physics, thanks for the heads-up everyone!

Elasticity and plasticity are mutually exclusive, correct? If a collision is perfectly elastic no plasticity can be exhibited and a perfectly plastic collision (ignoring plausibility) all of the energy would be used towards deformation.

Last edited: Jun 13, 2015
12. Jun 21, 2015

### caz

You are mixing the ideas of elastic/plastic material properties and elastic/plastic collisions. I am guessing that you are thinking about rigid body collisions in which elastic means that energy is conserved. Plastic is everything else.