Each of the three blocks P, Q and R shown in figure has a mass of 3 kg. Each of the wires A and B has cross sectional area `0.005 cm^2` and Young modulus `2xx10^11Nm^-2`. Neglect friction. Find the longitudinal strain developed in each of the wires. Take `g=10ms^-2`

Explanation

The block R will descend vertically and te blocks P and Q will move on the frictionless horizontal table. Let the common magnitude of the acceleration be a. Let the tensions in the wires A and B be `T_A and T_B` respectively.
writing the equations of motion of the blocks P, Q and R we get
`T_A=(3kg)a`........i
`T_B-T_A=(3kg)a`.............ii
`and (3kg)g-T_B=(3kg)a`............iii
by i and ii
`T_B=2T_A`
by i and iii
` T_A+T_B=(3kg)g=30N`
or `3T_A=30N`
`or, T_A=10N and T_B=20N`
`LongitudiN/Al strain =(longitudiN/Al stress)/(Young modulus)`
`Strain in wire A=(10N/0.005cm^2)/(2xx10^11Nm^-2)=2xx10^-4`