Question

A ball is moving in a straight line through a fluid which is spinning around its own centre of mass as shown.

Then, the ball experiences

• A. an upward force
• B. a downward force
• C. a leftward force
• D. no force at all

Answer 1

Answer: (A)

A ball which is spinning drags the fluid along with it. The given figure shows the streamlines of fluid and spinning at the same time.
The ball is moving forward and relative to it, the fluid is moving backwards.
Therefore, the velocity of the fluid above to the ball is larger and below is smaller. This difference in the velocity of fluid results in pressure difference between the lower and upper faces and there is a net upward force on the ball.