Experiment 5 – Time per oscillation of pendulum in liquids of different viscosities.

Aim of experiment: To find out how the time per oscillation of a pendulum differ when it is placed in liquids of different viscosities.

Hypothesis:

As stated in my summary of research, particularly with reference to Stoke’s Great Law of Drag:

Where

r = the radius of a sphere in cm

d1 = the density of the sphere in g/cm3

d2 = the density of the fluid in g/cm3

g = the local gravitational acceleration in cm/S2

c = the Viscosity of the fluid in Poise

v = the terminal velocity in cm/S

I can derive that the more viscous the fluid, the smaller the terminal velocity. Thus, I can form an hypothesis for my experiment (how does viscosity affect the movement of a pendulum in terms of speed?): The higher the viscosity of the liquid, the slower the pendulum will travel, therefore, the longer the time taken for one oscillation.

Materials Needed:

- 1 litre of water (viscosity 1)

- 1 litre of honey water (viscosity 10)

- 1 litre of honey water (viscosity 20)

- 1 litre of honey water (viscosity 30)

- 1 pendulum

- 1 measuring beaker (1 litre)

- 1 stopwatch

Experiment Procedure:

1. Fill the measuring beaker with water till it reaches the 1 litre mark.

2. Allow the honey to reach the temperature of the air-conditioned room (20ºC).

3. Use the thermometer to check the temperature.

4. Position the chopsticks above the beaker such that the pendulum (string included) is fully submerged.

5. Raise the pendulum till it is just touching the surface of the water.

6. Release the pendulum and start the stopwatch at the same time.

7. Stop the stopwatch as soon as one oscillation is completed.

8. Repeat the experiment 4 times more for more accurate results.

9. Repeat steps 1-8 with honey water (viscosity 10,20,30) instead of water.

Independent Variable: Viscosity of liquids the pendulum moves through.

Constant Variables:

1. Amount of mixture poured into beaker (1 litre).

2. Length of string of pendulum.

3. Weight of pendulum.

4. Temperature of liquids tested.

5. Height to which pendulum is raised to before release.

Dependent Variable: Time taken for the pendulum to complete one oscillation.

Results:

Liquid	Viscosity	Time/Osciliation (s)
Water (20ºC)	1	0.6
		0.68
		0.73
		0.72
		0.73
	Average time per oscillation	0.692
Honey Water (20ºC)	10
23% Honey		0.84
77% Water		0.87
		0.75
		0.84
		0.86
	Average time per oscillation	0.832
Honey Water (20ºC)	20	1.13
49% Honey		1.05
51% Water		1.19
		1.01
		1.16
	Average time per oscillation	1.108
Honey Water (20ºC)	30	1.38
74% Honey		1.42
26% Water		0.97
		1.3
		1.15
	Average time per oscillation	1.244

Data Analysis:

The honey water with viscosity of about 30 resulted in the longest time per oscillation of the pendulum, while water with viscosity of about 1 resulted in the shortest time. The time per oscillation increased as the viscosity of the liquids increased.

Conclusion:

The more viscous the liquid, the longer the time taken for the pendulum to complete one oscillation. This conclusion is valid only with the following assumptions:

1. The pendulum was raised to the same height before release, and there was no external force acting on it.

2. Human error of starting and stopping the stopwatch was minimal.