# The Common Balance Experiment (Class 11)

COMMON BALANCE EXPERIMENT FOR CLASS 11

Aim

To determine the mass of a rectangular block and hence find its density.

Apparatus

Beam balance, Weight box, Vernier callipers, the given body etc.

Principle

i. Sensibility of the balance. S = 0.01/(R0 —R1)

ii. Mass of the body, M= W + S (R—R0 )

iii. Volume of the rectangular block, V = l.b.t

where, l  - Length of the block

b - Breadth of the block

t - Thickness of the block

iv. Density = Mass/Volume = M/V

Procedure

To begin with, the pillar is made vertical by adjusting the levelling screws (S1 and S2). The beam is released gently by turning the handle with pans empty. The oscillations are observed. The nuts (n1 and n2) on the beam are adjusted in such a way that the pointer moves to and fro over the scale equally from zero graduation. The beam is arrested.

a. Sensibility Method

The balance is gently released. Five successive turning points starting from the left are taken. The average of the three turning points on the left and the average of the two turning points on the right are found. The average of these two averages gives the zero resting point (Ro) of the balance.

A load of 10 mg is placed the right pan. The balance is gently released and the new resting (R1) is found as before. The change in resting point due to 10 mg is R0-R1. Therefore sensibility, S = 0.01/(R0-R1) can be calculated.

The body is placed in the left pan. Sufficient weights are placed in the right pan, so that the pointer swings almost equally to both sides. As before five successive turning points starting from the left are taken and the resting point (R) is determined. The total mass (W) in the right pan is noted. Hence the correct mass (M) of the body can be calculated.

b. Equal Oscillation Method

The given body is placed on the left pan. Weights from the weight box are placed on the right pan till the pointer moves to and fro over the scale equally from the zero graduation on releasing the beam. The weight ‘m1’ on the right pan is recorded. The experiment is repeated by placing the body on the right pan and weights on the left pan. The weight ‘m2’ on the left pan is noted. The average value of these two is calculated. This gives the mass 'M' of the given body.

Determine the length (l), breadth (b) and thickness (t) of the block using vernier callipers. Hence volume of the block can be calculated using the formula V = lbt.

Therefore, Density of the material of the block = Mass/Volume

To find the weight of the body

Sensibility, S = 0.01/(R0-R1) = .0095 g/div

Mass of the body, M = W + S(R – R0) = 5.7198 g = 5.7198 x 10-3 kg

Results

The mass of the body = 5.7198 x 10-3 kg

MODEL VIVA VOCE QUESTIONS AND ANSWERS

1. Define mass and weight.

Mass of a body is the amount of matter contained in it. Weight an body is the force with which the earth attracts the body towards its centre.

2. What is the S.I. unit of mass?

kilogram (kg)

3. What is the S.I. unit of weight?

newton (N)

4. Mention some differences of mass and weight.

i. Mass is a scalar quantity whereas weight is a vector quantity.

ii. Mass of a body is a constant whereas weight is not a constant. It is different at di ',wail places.

5. How do you measure mass?

Mass is measured using a common balance.

6. How do you measure weight?

Weight is measured using a spring balance.

7. What is the unit used for measuring mass of atom or subatomic particles?

Atomic mass unit

8. What is meant by density of a substance?

It is the ratio of mass to the volume of a body.

9. What is the standard value of density of water at 4° C?

Density of water = 1000 kg/m3

10. While weighing, you usually close the shutter of the balance. Why?

While weighing, we usually close the shutter of the balance in order to avoid air current.

11. Why are the knife-edges and plates made of agate?

To reduce friction

12. Is it advisable to weigh a hot body?

No

13. Why do you put standard masses in the right-hand pan and the body in the left hand pan?

Standard masses are put in the right hand pan for convenience. (Masses can be adjusted more frequently).

14. Define relative density.

Relative density is defined as the ratio of the density of a substance to the density of water at 4°C.

15. What is meant by the sensibility of the balance?

It is the mass required to change the resting point by 1 division.

16. What are the requisites of a good balance?

Truth, Sensibility and Stability