Understanding Stress & Strain in 9th Standard Physics

[jayanth nivas]

Hi all,
This is my first post in PF
I'm doing my ninth standard in India,My knowledge in physics is limited so please forgive me if I ask something basic

Can anyone clearly explain me the relationship stress and strain,because I thought when a material deforms (even to the slightest degree)restoring forces are set up to bring it back to original position (which is stress in elastic zone).

By when a bar constrained at both ends is heated,it cannot deform because it is fixed,whereas it is stresses(which violates what i said before because stress can occur only if there is strain).

Once again sorry if i have asked something very basic and thanks in advance.

 

[SteamKing]

If the heated bar were not restrained in any way, after heating, a measurement of the bar would show that its length had increased. The bar expands in response to the heating.

If the bar is fixed such that no increase in length is possible, then application of heat to the bar cannot cause an increase in length but it can cause an increase in the stress of the bar. The increased stress is of a compressive nature, which counteracts the bar's tendency to increase in length due to heating. The amount of stress generated, if applied to a bar which was not restrained, would cause the bar to contract its length, by an amount which would be equal to the expansion which would be caused by heating.