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.

Question 17

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.

Reference: Past Exam Paper – June 2015 Paper 13 Q15

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.