Page 129 - Physics - XI

Principle/Theory

Resonance: The phenomenon of setting a body A into vibrations with its natural frequency with the help
of another body B vibrating with the same frequency is called resonance. The vibrations so produced in the
body A are called resonant vibrations.
When a stretched string of a sonometer is set in vibrations, the frequency n of the fundamental mode of
vibration of the string is

n = 1 T
2l m
where l is the length of the string between the wedges.
T is the tension in the string.

m is the mass per unit length of the string.
For a given m and fi xed T,
1
n ∝ or nl = constant
l
1
If we plot a graph between n and , the graph will be a straight line. If we plot a graph between n and l,
l
the graph will be a rectangular hyperbola.

Procedure
1. Fix the sonometer on the table and check whether the pulley is frictionless or not. If not, oil it. Stretch
the wire by placing a suitable load on the hanger.

2. Place two equal size sharp bridges below the wire close to each other. Cut a small piece of paper and
fold it into V-shape, it is called a rider. Place the rider on the wire in the middle of the two bridges.

3. Set the given tuning fork into vibration by striking its prong gently against the rubber pad and place its
stem fi rmly on the sonometer board keeping the tuning fork vertical. Go on increasing the length of the
vibrating wire by adjusting the position of the two bridges till the paper rider starts vibrating and fi nally
falls down. (Always keep the rider in the middle of the two bridges.) In this position, the frequency
of the wire is exactly equal to that of the tuning fork. Measure the length of the wire between the two

bridges with the help of a metre scale, this is the resonant length of the wire while increasing.
4. Increase the distance between the bridges by 4 to 5 cm by moving them outward so that the length of
the wire is increased. Repeat step 3 by decreasing the length of the wire till the paper rider falls down.
Note the distance between the two bridges with the help of a metre scale. This is the resonant length
of the wire while decreasing.

5. Repeat steps 3 and 4 with fi ve more tuning forks of diff erent frequencies with the same wire and same
load (tension).
6. Note all the observations in the observation table.

Observations
Constant value of load on the wire = M kg
Constant value of tension on the wire = T = Mg = _____ N.

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