Q#17 (Past Exam Paper – June 2015 Paper 13 Q15)
An air bubble in a tank of water is rising with constant velocity. The forces acting on the bubble are X, Y and Z as shown.
What describes the three forces?
A Z is the viscous drag on the bubble, Y is the weight of the bubble, X is the upthrust on the bubble and X = Y + Z.
B Z is the viscous drag on the bubble, Y is the weight of the bubble, X is the upthrust on the bubble and X > Y + Z.
C Z is the weight of the bubble, Y is the viscous drag on the bubble, X is the upthrust on the bubble and X = Y + Z.
D Z is the weight of the bubble, Y is the viscous drag on the bubble, X is the upthrust on the bubble and X > Y + Z.
Solution:
Answer: A.
The air bubble rises at constant velocity. This means that its acceleration is zero, and thus the resultant force on it is also zero [since F = ma].
Resultant force = 0
Upward force = Downward forces
X = Y + Z [Choices B and D are incorrect]
The weight acts downwards. So, either Y or Z is the weight.
BUT the mass of air is very tiny. Its weight will also be very small. So, Y is most likely the weight.
Z is the viscous drag of the water on the bubble. The viscous drag is a resistive force and therefore acts in a direction opposite to the motion.
X is the upthrust force which arises due to the difference in pressure at the top and bottom of the air bubble. Its direction is upwards.
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