A `20 N` metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads `40 N`. The spring balance is now lowered so that the block gets immersed in the water. The spring balance now reads `16 N`. The reading of the weighing machine will be.

Explanation

The upward thrust of the metal block = `20 - 16 = 4 N`, Ac cording to Newton's third law of motion, it will exerts a force of 4N on the weighing machine. Thus the reading of the weighing machine = ` 36 + 4 = 40 N`.