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Practical investigation into Viscosity in liquids (Stokes Law).

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Practical investigation into Viscosity in liquids(Stokes Law)


When dealing with fluid/mechanical systems, it is important to know what affects the rate of descent of an object through a liquid.

There are many factors that affect the descent of an object through a liquid such as:

1) Temperature of the liquid

2) Mass* of object

3) Size/surface area of object

4 Viscosity of liquid

5) Angle of descent


I would like to investigate the correlation between temperature and time of descent. Reading suggests that the colder the liquid the longer it will take for the object to reach the bottom.

Mass*& Surface area/size

Gravity accelerates at 9.81 ms-1 independent of mass. Hence increasing the mass will not affect the experiment of surface area. Thus using an object of various sizes it would be possible to investigate the proportionately of size on the descent of the object.


I feel it is important to investigate the affects of how a more viscous liquid would impede the progress of an object descending through a liquid. Therefore I have included this factor into my investigation.

Angle of descent

I would like to observe the affects of the object descending at an angle. Such at sediment in a bottle is there a way in which bottles should be stored that may hasten descent?


...read more.


Now that I had my first set of results, I thought I would introduce a variable of five differently sized ball bearings. This enabled me to investigate the surface area, mass* and how this effected the rate of descent.

The results of experiment 1 are as follows:

Very Small
Distance Timed (cm) Time taken(s) for ball bearing to pass through distance measured
1 2 3 Average
0-20 0.19 0.21 0.20 0.200
10-30. 0.17 0.18 0.20 0.183
20-40 0.18 0.19 0.20 0.190
30-50 0.20 0.18 0.19 0.190

Distance Timed (cm) Time taken(s) for ball bearing to pass through distance measured
1 2 3 Average
0-20 0.24 0.25 0.24 0.243
10-30. 0.20 0.21 0.21 0.206
20-40 0.18 0.18 0.19 0.183
30-50 0.17 0.19 0.18 0.180
40-60 0.18 0.18 0.18 0.18
50-70 0.17 0.18 0.19 0.18

Distance Timed (cm) Time taken(s) for ball bearing to pass through distance measured
1 2 3 Average
0-20 0.32 0.33 0.32 0.323
10-30. 0.28 0.28 0.29 0.283
20-40 0.23 0.23 0.22 0.227
30-50 0.19 0.21 0.20 0.200
40-60 0.21 0.20 0.20 0.203
50-70 0.20 0.20 0.21 0.203

Distance Timed (cm) Time taken(s) for ball bearing to pass through distance measured
1 2 3 Average
0-20 0.26 0.28 0.28 0.273
10-30. 0.23 0.22 0.20 0.217
20-40 0.20 0.21 0.20 0.203
30-50 0.20 0.19 0.20 0.197
40-60 0.20 0.20 0.20 0.200
50-70 0.20 0.19 0.2 0.197

Very Large
Distance Timed (cm) Time taken(s) for ball bearing to pass through distance measured
1 2 3 Average
0-20 0.33 0.32 0.30 0.317
10-30. 0.28 0.29 0.27 0.280
20-40 0.27 0.26 0.26 0.263
30-50 0.23 0.24 0.25 0.240
40-60 0.22 0.23 0.23 0.227
50-70 0.22 0.22 0.22 0.220

Experiment 1.2

The second part of the first experiment measured the time taken for five differently sized ball bearings to descend through water. The purpose of this part of the experiment was to make it more clear how surface area and mass* affects the rate of descent.

At this point and introduction of a sixth ball was used.

...read more.


3) I have found that the angle in which the ball bearing descends through will decrease the speed of when it will reach the bottom. However as I mentioned in the discussion an extra force was acting upon this and therefore made this experiment invalid.

4) The final conclusion to be drawn from my investigation, is that the ball bearings seemed to reach their terminal velocity in the same timed interval. For experiment 1 it was 40-60 cm and for experiment 2 this was also 40-60. Therefore I would be able to conclude with a third liquid that it may be possible that the liquid does not effect the point in which a ball bearing reaches its terminal velocity.

However I can conclude that the size of the ball bearing and also the mass does not effect where it reaches its terminal velocity. As you already know, if two objects of the same size but with different masses are dropped from the same height they will descend and hit the ground at the same time. It is only air resistance that will affect the descent if the objects size is slightly different. I can relate this to my experiments in finding the terminal velocity of the ball bearings through the liquid, and therefore explain why the occurance happened with only a slight varience with the very large ball bearings.

...read more.

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