Experiment to study the fall of a sphere through a viscous medium, measure terminal velocity, and calculate the coefficient of viscosity with precautions.
Apparatus
Long, hard-glass tube closed at the bottom and fitted vertically on a stand, engine oil or glycerin, marker, plumb line, meter rule, screw gauge, digital stopwatch, and steel balls.
Procedure
First, observe and record the room temperature because viscosity is greatly affected by temperature. Set the glass tube vertically using a plumb line and adjust it with the leveling screws.
Next, mark horizontal lines on the tube. Draw a line D about 5 cm above the bottom, then another line C 10 cm above D. Similarly, mark two more lines B and A, each 10 cm apart. This divides the lower portion of the tube into three equal segments.
Now, fill the tube with engine oil or glycerin nearly up to the top. Take a small steel ball and measure its diameter with a screw gauge. Record the diameter and calculate the radius in the observation table.
Hold the stopwatch in one hand and gently release the steel ball into the liquid. Start timing as the ball crosses mark A, press the lap button when it crosses B, and note the time. Continue this process as the ball passes C and D. The times between these marks should be nearly equal if the ball has reached terminal velocity.
Find the terminal velocity of the ball using the given formula and record it. Repeat the experiment several times with steel balls of different diameters. Then, calculate the mean terminal velocity and use the relation provided to find the coefficient of viscosity.
Finally, note the room temperature again and calculate the mean temperature.
Observations and Calculations
Result: 
Precautions
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The apparatus should be perfectly vertical.
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Release the sphere gently without any push.
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Time must be noted carefully for accuracy.
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Avoid air bubbles in the liquid.
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Experiment with a constant temperature.
Viva Voce
Q1: Define terminal velocity.
Ans: It is the constant velocity attained by a body moving through a fluid.
Q2: How can the terminal velocity of a body be decreased? Give an example.
Ans: By increasing its effective radius, e.g., a parachute.
Q3: How is viscosity affected by temperature?
Ans: With an increase in temperature, the viscosity of liquids decreases while that of gases increases.
Q4: What is viscosity?
Ans: The internal resistance between two adjacent layers of a liquid is called viscosity.
Q5: What is the SI unit of viscosity?
Ans: The SI unit of viscosity is N·s/m² (Newton second per square meter).
Q6: What is a free-falling body?
Ans: A body that falls under the action of gravity alone is called a free-falling body.