Equilibrium in elastic collisions

Question 19 states that the collision is an elastic collision, so momentum and kinetic and energy are both conserved. This means therefore that when disc X hits disc Y , Y will stay still and Z will move away to the right with the same speed and energy as X had originally assuming they all have equal mass (a bit like a Newton's cradle). So, for Y to remain in its original position, the forces on it must remain balanced.

Question 20 - the volume of air inside the tubing is not being heated - only that inside the flask. Therefore you keep the tubing volume as low as possible so that it will not add much to the value of V. There will be a delay between the temperature of the water reaching a set point and the heat transferring to the air in the flask. You really need to get the water to say, 30C and keep it there for a wee while then measure the pressure. then do the same at 40C etc. Immersing the largest possible amount will transfer the heat from the water to the air in the flask better. You also want all the air to be at the same temperature i.e no heat gradient in the flask