# 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/(R_{0} —R_{1})

ii. Mass of
the body, M= W + S (R—R_{0} )

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 (S_{1}
and S_{2}). The beam is released gently by turning the handle with pans
empty. The oscillations are observed. The nuts (n_{1} and n_{2})
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 (R_{o}) of the balance.

A load of
10 mg is placed the right pan. The balance is gently released and the new
resting (R_{1}) is found as before. The change in resting point due to
10 mg is R_{0}-R_{1}. Therefore sensibility, S = 0.01/(R_{0}-R_{1})
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 ‘m_{1}’ 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 ‘m_{2}’ 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

**Observation Table ****and Readings**

To find the
weight of the body

Sensibility,
S = 0.01/(R_{0}-R_{1}) = __.0095__ g/div

Mass of the
body, M = W + S(R – R_{0}) = __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/m^{3}

*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

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