Question

A wooden piece is balanced by a brass weight in a sensitive balance. Now the pan contained by wooden piece is covered by a bell jar and then evacuated;
A) The pan containing the wooden piece will go up
B) The pan containing the wooden piece will go down
C) The beam will remain horizontal
D) Nothing can be said definitely

Answer 1

The balance works on the principle of moments at equilibrium. According to this principle, when a body is balanced, the total clockwise moment about a point is equal to the total anti clockwise moment about the same point.

Complete step-by-step answer:
Step I:
Moment is defined as the turning effect of force. When no force is acting on the system then it is said to be in equilibrium. This is because all the forces will cancel each other.
Step II:
The beam consists of two pans at equal distances from the center of the beam. The object whose mass is to be measured is kept on one pan and the other pan is balanced by setting brass weights.
Step III:
Given that the wooden piece is balanced by a brass weight with a sensitive balance. Therefore, now the balance is in equilibrium. But now the pan in which the wooden piece is kept is covered by a bell jar and then evacuated. Then again the balance will be in equilibrium, since both sides of the pan are evacuated. Hence the beam will remain horizontal.

Therefore, Option C is the correct answer.

Note:
Sometimes the balance used for measuring the weights can be faulty. A faulty beam balance has pans that have different mass. When the pans are empty, the beam remains horizontal. If a mass is kept on one pan, it will show different weight. If it is kept on another pan, then the weight will be different.