
Obtain an expression for the period of a simple pendulum. On what factors does it depend on?
Answer
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Hint: To derive the time period of a simple pendulum. We first find all the forces acting on the pendulum for the oscillation to take place. From the force formula we find an equation for acceleration. Comparing this equation with the acceleration equation in simple harmonic motion we find the time period.
Complete step by step solution:
The free body diagram of a simple pendulum is as given below

The tension force is represented by
Weight of the bob is represented by
Gravity is represented by
Mass of bob is represented by
The horizontal component of weight is represented by
Vertical component of weight is represented by
Angle is the angle made by the initial position with the extreme position. This is also equal to the angle between weight and its horizontal component from internal angles.
At the initial position there are no components for the weight of Bob. At the extreme position the weight of the Bob is divided into vertical and horizontal components as in the diagram. The tension force and the vertical component of the weight cancel each other.
And the remaining force acting on the pendulum to cause oscillation is
Negative sign represented the direction.
Let us take the length of the pendulum as
Radius of Bob as
Total length is represented by
Taking the diagram as a right angled triangle
From the diagram is
Where is the displacement of bob
Now the force becomes
Acceleration is equal to
Taking as constant we get
.....(1)
From shm acceleration is equal to
......(2)
Equating (1) and (2)
We get
is angular frequency which is equal to
Here, represented time period
Substituting this in the above equation we get
Hence time period is
From the final equation we can say that the time period of a pendulum depends on length and acceleration due to gravity.
Note: We can consider the oscillations in a pendulum as simple harmonic motion because if a graph of the displacement of the Bob and the time is drawn it shows a simple harmonic motion. Hence, we can say that an oscillating pendulum is a simple harmonic motion.
Complete step by step solution:
The free body diagram of a simple pendulum is as given below

The tension force is represented by
Weight of the bob is represented by
Gravity is represented by
Mass of bob is represented by
The horizontal component of weight is represented by
Vertical component of weight is represented by
Angle
At the initial position there are no components for the weight of Bob. At the extreme position the weight of the Bob is divided into vertical and horizontal components as in the diagram. The tension force and the vertical component of the weight cancel each other.
And the remaining force acting on the pendulum to cause oscillation is
Negative sign represented the direction.
Let us take the length of the pendulum as
Radius of Bob as
Total length is represented by
Taking the diagram as a right angled triangle
Where
Now the force
Acceleration is equal to
Taking
From shm acceleration is equal to
Equating (1) and (2)
We get
Here,
Substituting this in the above equation we get
Hence time period is
From the final equation we can say that the time period of a pendulum depends on length and acceleration due to gravity.
Note: We can consider the oscillations in a pendulum as simple harmonic motion because if a graph of the displacement of the Bob and the time is drawn it shows a simple harmonic motion. Hence, we can say that an oscillating pendulum is a simple harmonic motion.
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