Page 113 - Physics - XI

If v is the terminal velocity of the sphere then according to Stoke's law,

upward viscous force = F = 6 πηrv.
When body attains terminal velocity then

Upward force = Downward force

4
6πηrv = r ( g)
3
3
2r ( ) g
2
terminal velocity, v =
9
Procedure
1. Clean the long glass cylinder and fi x it with its open end upward with the help of a clamp stand. Fit

a rubber cork in the mouth of the cylinder after fi lling it with glycerine and pass the small glass tube
through it. Keep the liquid bubble free.
2. With the help of a thermometer, note down the room temperature.
3. Put three rubber bands A, B, and C around the glass cylinder dividing it into four portions such that
AB = BC each about 30 cm.
4. Take a steel ball-bearing of such a size so that it can pass easily through the glass tube. Measure its
diameter with the help of the screw gauge.
5. Take a steel ball and dip it in a small quantity of glycerine in a watch glass so that the surface of the
ball is thoroughly coated with glycerine.

6. Now, take the steel ball from the watch glass and gently drop it in the small glass tube so that it falls
centrally through the glass cylinder as shown in Fig. 5.2. After moving a few centimetres through the
glycerine, the ball will attain a uniform terminal velocity. When the balls passes down the upper edge
of the fi rst rubber band A, start the stopwatch and stop it when it reaches the upper edge of the next
rubber band B.

7. When the ball crosses C, stop the second stopwatch.
8. Note the times t and t as recorded by the two stopwatches.
1 2
9. Repeat the steps 4, 5, 6, 7, and 8 by dropping four more balls of diff erent sizes.
10. Record all the observations in the table.

Observations
1. Internal diameter of the long glass cylinder = ______ cm = ______ m
2. Length of the long glass cylinder = ______ cm = ______ m

3. Distance between A and B = ______ cm = ______ m
4. Distance between B and C = ______ cm = ______ m
Average distance h between two consecutive rubber bands = ______ cm = ______ m

5. Acceleration due to gravity at the place of experiment = ______ms –2
6. Least count of stopwatch = ______ s
7. Temperature of experimental liquid (glycerine), T = ______ °C
8. Density of material of steel balls, σ = ______ kg m –3

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