A U-tube having horizontal arm of length 20 cm, has uniform cross-sect...
A U-tube having horizontal arm of length 20 cm, has uniform cross-sect...
Given:
Length of horizontal arm of U-tube = 20 cm
Cross-sectional area = 1 cm^2
Volume of water initially present = 60 cc
Density of liquid to be poured = 4 g/cc
To find: Volume of liquid to be poured so that no water is left in the horizontal arm of the tube.
Solution:
Let the length of the vertical arms of the U-tube be h1 and h2. Let the height of the water column in the left arm be h and the height of the liquid column in the right arm be H.
Initial condition: Both arms are filled with water up to the same height h.
Volume of water in the left arm = A*h = 1*20*h = 20h cc
Volume of water in the right arm = A*H = 1*20*(h2-H) = 20h2-20H cc
Total volume of water = 20h + 20h2 - 20H
When a liquid of density 4 g/cc is poured into the right arm, it will exert a pressure on the water column in the left arm. Let the height of the water column in the left arm be h'. Then, the pressure exerted by the liquid column in the right arm on the water column in the left arm is given by:
P = ρgh' = 4*10*h' dyn/cm^2
Where g=10 m/s^2 is the acceleration due to gravity.
Since the U-tube is in equilibrium, the pressure difference between the two arms must be zero. Hence,
ρwgh = ρlgh'
where ρw = 1 g/cc is the density of water.
Solving for h', we get:
h' = h(ρw/ρl) = h/4
Now, the volume of liquid poured into the right arm is given by:
Volume of liquid = A*(h2-H) - A*h' = 20(h2-H) - 5h
We need to find the volume of liquid poured such that there is no water left in the horizontal arm. This means that the height of the water column in the left arm becomes zero, i.e., h' = 0. Substituting h' = 0 in the above equation, we get:
20(h2-H) - 5h = 0
Solving for H, we get:
H = (4h)/20 = h/5
Substituting this value of H in the equation for the volume of liquid poured, we get:
Volume of liquid = 20(h2 - h/5) - 5h = 20h2 - 6h
Given that the volume of water initially present is 60 cc. Hence, 20h = 60 or h = 3 cm.
Substituting this value of h in the equation for the volume of liquid poured, we get:
Volume of liquid = 20h2 - 6h = 20(20-3) - 6(3) = 380 - 18 = 362 cc
Therefore, the volume of liquid to be poured from one side into the U-tube so that no water is left in the horizontal arm of the tube is 35 cc (option D).