Elasticity Question!

Q. Two balls of same mass, volume, size, etc are thrown to two different mirrors. One mirror has already a stress applied on it and the other one has no stress on it. Which mirror is most likely to break? Explain.

Lets say the ball gives a stress to the mirror called the 'ball stress':



.............................................Stress before ball hits...........Stress During ball impact

Unstressed Mirror.....................Stress 1 = 0..........................Ball Stress

Stressed Mirror........................ Stress 2 > 0..........................Stress 2 + Ball Stress



Stress 2 + Ball Stress > Ball Stress

Which stress is higher? What does this mean?

Sorry for the weird format, TSR is crap

No problem. So do you mean that the stressed mirror is most likely to break?

You tell me! Really, what do you think? I'm sure you will be correct!

LOL! I don't know whether I'm right Or wrong. According to me the glass which has already been applied with stress should break.

But the question with SO SIMPLE answers and statements don't come in A level. What do you think? - am I right?

Yes you are!

The higher the stress the more likely it is to break. Maybe you should revise the basics of what a stress is.

Stress = Force applied / Area over which it is applied

So a larger stress corresponds to a higher force, if the areas of the two mirrors are the same. A higher force means the mirror is more likely to break, doesn't it?

A stress is similar to a force, it just takes into account the area over which it is applied.

If I put a big book on your head, it wont hurt, because the force from the weight of the book is spread out over the top of your head and the stress is low.

However if I put a pin on the bottom of the book and put that on your head, the area is thousands of times smaller, and it is going to hurt!

Why? Because even though the force has barely changed (the pin weighs hardly anything), the area over which it is applied has changed, and the stress has increased.

The high stress is what hurts, and it's the same for the mirror. High stresses cause damage!

High forces only cause damage because they lead to high stresses. Get it?

You can think of it in terms of a person too, a person who is stressed out is going to be closer to 'breaking point' than somebody that has no stress.