Answer: first statement, "At segment T-U, the substance changes from liquid to gas and does not change temperature"
Explanation:
if the segment T-U is flat (not inclined), means that the temperature is not changing. In this case, the heat provided to the substance is being used to overcome the intermolecucular attraction forces, which permits the particles to get more separated and, in such way, the liquid pass to gas phase.
At segment T-U, the substance changes from a liquid to a gas and does not change temperature.
The reason is because latent heat of vaporisation allows for the absorption of heat in the change of state and temperature remains constant until it has fully changed state.
Given there are three blocks of masses , and (ref image in attachment)
When all three masses move together at an acceleration a, the force F is given by
F = ( + + ) *a ................(equation 1)
Also it is given that does not move with respect to , which gives tension T is exerted on pulley by only, Hence tension T is
T = *a ..........(equation 2)
There is also also tension exerted by . There are two components here: horizontal due to acceleration a and vertical component due to gravity g. Thus tension is given by
T = ................(equation 3)
From equation 2 and 3, we get
*a =
Squaring both sides we get
* = * (+)
* = ( * )+ ( *)
( - ) * = *
= */( - )
Taking square root on both sides, we get acceleration a
a = *g/()
Hence substituting the value of a in equation 1, we get