Finger in the Water

A bucket of water sits on a scale. You slowly stick your finger in the water and hold it there without touching the sides of the bucket. Does this change the reading on the scale?

A) Yes, it increases.

B) No.

C) Yes, it decreases.

D) It depends or I can't tell.

Solution: A. The finger displaces a volume of water. The water exerts an upward buoyant force on the finger by Archimedes' principle. The finger exerts a downward force on the water by Newton's third law. The water exerts a stronger force on the bucket and the bucket exerts a larger force on the scale by Newton's second law.