Question

A flask of water rests on a scale that reads 100 N. Then, a small block of unknown material is held completely submerged in the water. The block does not touch any part of the flask, and the person holding the block will not tell you whether the block is being pulled up (keeping it from falling further) or pushed down (keeping it from bobbing back up). Assume that no water spills out of the flask. What is the new reading on the scale?
a) greater than 100 N
b) 100 N
c) less than 100 N
d) It is impossible to determine.

Answer 1

Answer:

The option (a) is correct.  

Explanation:

The expression for the buoyant force is as follows;

$F=\rho Vg$

Here, F is the buoyant force, V is the volume and g is acceleration due to gravity.

According to the given problem, a flask of water rests on a scale that reads 100 N. Then, a small block of unknown material is held completely submerged in the water.

The weight of the system acts in the downward direction and the reading on the scale shows downward normal force applied by the block. From Newton's third law, the scale applies an upward normal force on the block. And the third force acting on the block is buoyant force.

The upward force acting on the block which is exerted by the fluid. This force is called buoyant force. The new reading on the scale will increase due to the buoyant force. The scale reading will be greater than 100 N.

Therefore, the option (a) is correct.