Unit 1 Engineering Physics

1.7. TORSION PENDULUM

m m m m

d 1

d 1

d 2

d 2

(i) (ii) (iii)

Axis of cylindrical

mass

Figure 1.22: Determination of Rigidity Modulus - dynamic torsion method

and using the relation(5.297) we obtain,

T 02

T 22 ≠T 12

=

I

I 2 ≠I 1

=

I

2 m(d^22 ≠d^21 )

Hence,

I=

2 m(d^22 ≠d^21 )T 02

T 22 ≠T 12

(1.23)

Substituting from equation (5.298) into the expression (5.293) for Rigidity Modulus, we
arrive at,

n=

8 fiIl

T 02 r^4

=

8 fil

T 02 r^4

A

2 m(d^22 ≠d^21 )T 02

T 22 ≠T 12

B

Hence, the Rigidity Modulus is also given by:

n=

16 fiml(d^22 ≠d^21 )

r^4 (T 22 ≠T 12 )

(1.24)

Therefore, from the experimental data obtained by measuring the massmof one of the
bodies placed on the disc, the distancesd 1 andd 2 at which they are placed from the disc
centre, the periodsT 1 andT 2 of oscillation and the radiusrof the suspension wire, the
Rigidity Modulus of the wire can be calculated.

Self Learning Activity : Torsion Pendulum

Using the torsion pendulum simulator, study the behaviour of

dierent torsion pendula and determine moments of inertia of

the given discs and rigidity moduli of the available materials.

Also

http://vlab.amrita.edu/?sub=1&brch=280

PH8151 25 LICET