Physics

Simple pendulum

Chapter: Oscillations and Waves

Question 1 of 5 NDA MCQ

The time period of a 1 m long pendulum approximates to

Detailed Explanation

Correct Answer

โœ“ Option (c) is correct. The time period of a 1 m long pendulum approximates to 2 seconds.

๐Ÿ“– Governing Law / Core Concept

The time period \( T \) of a simple pendulum is governed by the formula:
\[ T = 2\pi \sqrt{\frac{L}{g}} \]
where:
  • T = Time period (seconds)
  • L = Length of the pendulum (meters)
  • g = Acceleration due to gravity (approximately 9.81 m/sยฒ)

๐Ÿงช Step-by-Step Breakdown

In this case, let \( L = 1 \) m. Substitute \( L \) into the time period formula:
\[ T = 2\pi \sqrt{\frac{1}{9.81}} \]
Calculate \( \sqrt{\frac{1}{9.81}} \):
\[ \sqrt{\frac{1}{9.81}} \approx 0.318 \]
Now, find \( T \):
\[ T \approx 2\pi \cdot 0.318 \approx 2.0 \text{ s} \]
Thus, the time period approximates to 2 seconds.

๐Ÿ” Option Analysis

- Option (a) 6 s: Too long for a 1 m pendulum. - Option (b) 4 s: Exceeds expected range for this length. - Option (d) 1 s: Too short, inconsistent with calculations.

โšก Mnemonic / Speed-Run

For estimating pendulum time periods, use \( T \approx 2\sqrt{L} \) when \( g \) is considered as approximately 10 m/sยฒ for quick mental calculations.

6. Visual Suggestion

6. Visual Suggestion

A diagram depicting a simple pendulum, showing the length \( L \), the pivot point, and the path of the bob swinging, aiding in visualizing the concept of the oscillation and its time period.

๐Ÿ“– Factual Verification & Reference

๐Ÿ“– Factual Verification & Reference:

Verified against NCERT Class XII Physics, Chapter 15 (Oscillations).

Question 2 of 5 NDA MCQ

A simple pendulum having bob of mass m and length of string l has time period of T. If the mass of the bob is doubled and the length of the string is halved, then the time period of this pendulum will be

Detailed Explanation

Correct Answer

โœ“ Option (b) is correct. The time period of the pendulum will be \(\frac{T}{\sqrt{2}}\).

๐Ÿ“– Governing Law / Core Concept

The time period \(T\) of a simple pendulum is given by the formula:

\[ T = 2\pi \sqrt{\frac{l}{g}} \]

where \(l\) is the length of the pendulum and \(g\) is the acceleration due to gravity. Notably, the mass \(m\) of the bob does not affect the time period of the pendulum.

๐Ÿงช Step-by-Step Breakdown

Initially, let the length be \(l\) and the time period be \(T\):

\[ T = 2\pi \sqrt{\frac{l}{g}} \]

Now, the mass of the bob is doubled (\(2m\)) but does not impact \(T\). The length of the string is halved, so we now have:

\[ l' = \frac{l}{2} \]

The new time period \(T'\) becomes:

\[ T' = 2\pi \sqrt{\frac{l'}{g}} = 2\pi \sqrt{\frac{\frac{l}{2}}{g}} \]

Simplifying this gives:

\begin{aligned} T' & = 2\pi \sqrt{\frac{l}{2g}} \\ & = \frac{1}{\sqrt{2}}(2\pi \sqrt{\frac{l}{g}}) \\ & = \frac{T}{\sqrt{2}} \end{aligned}

Thus, the new time period is \(\frac{T}{\sqrt{2}}\).

๐Ÿ” Option Analysis

  • Option (a): \(T\) โ€“ incorrect; mass change does not affect the period.
  • Option (c): \(2T\) โ€“ incorrect; halving length decreases period.
  • Option (d): \(\sqrt{2} T\) โ€“ incorrect; also inconsistent with length reduction.

โšก Mnemonic / Speed-Run

Remember, the mass of the pendulum does not affect the time period; only the length does. "Length down, period down but with a factor of \(\sqrt{2}\)."

๐Ÿ“Š Visual Suggestion

Visual Suggestion

Schematic diagram showing a simple pendulum with labeled length \(l\) and bob mass \(m\). Include two scenarios: the original pendulum and the modified pendulum (reduced length and increased mass).

๐Ÿ“– Factual Verification & Reference

๐Ÿ“– Factual Verification & Reference:

Verified against standard physics textbooks and NCERT Class XI Physics, Chapter 15 (Oscillations).

Question 3 of 5 NDA MCQ

Which one of the following statementsregarding simple pendulum is correct? Simple pendulum has a timeperiod independent of amplitude :

Detailed Explanation

Correct Answer

โœ“ Option (d) is correct. The time period of a simple pendulum is independent of amplitude only for small displacements.

๐Ÿ“– Governing Law / Core Concept

The time period \( T \) of a simple pendulum is governed by the formula:

\[ T = 2\pi \sqrt{\frac{l}{g}} \]

where:

  • T = period in seconds (s)
  • l = length of the pendulum in meters (m)
  • g = acceleration due to gravity in meters per second squared (m/sยฒ)

๐Ÿงช Step-by-Step Breakdown

For small angular displacements, the gravitational force provides a restoring force proportional to displacement. Thus:

Using the small angle approximation, the equation of motion becomes:

\[ F \approx -kx \text{, where } k = \frac{mg}{l} \]

For small amplitudes:

The restoring force is proportional to the displacement:

\[ F = -\frac{mg}{l}x \]

This leads to simple harmonic motion with a constant time period \( T \), independent of amplitude.

๐Ÿ” Option Analysis

  • Option (a): Incorrect - Only small amplitudes yield independent time periods due to linear approximation.
  • Option (b): Incorrect - Only small angles support a consistent time period due to proportionality in F.
  • Option (c): Incorrect - Time period is not independent for larger amplitudes.

โšก Mnemonic / Speed-Run

Remember: "For small swings, time rings are the same." This helps recall that small displacements yield constant periods.

6. Visual Suggestion

6. Visual Suggestion

A clean diagram of a simple pendulum, illustrating its length \( l \), amplitude \( A \), and angle \( \theta \) with the vertical. Include force vectors for gravity and tension to elucidate net force perpendicular to motion.

๐Ÿ“– Factual Verification & Reference

๐Ÿ“– Factual Verification & Reference:

Verified against NCERT Class XI Physics, Chapter 15 (Oscillations).

Question 4 of 5 NDA MCQ

The length of a simple pendulum is increased four times to its previous value while the mass is doubled. What is the ratio of the new and previous time period of the pendulum?

Detailed Explanation

๐Ÿ’ก Solution

t:T 2:1

๐Ÿ’ก Hints & Visual Guide

๐Ÿ“Œ Hints / Properties Used:

  • Time period of a simple pendulum is given as \( T = 2\pi \sqrt{\frac{l}{g}} \).
  • Mass of the bob does not affect the time period.

๐Ÿ“Š Visual Diagram Suggestion:

[A labeled diagram of a simple pendulum indicating the length and its properties, including a side-by-side comparison of the time periods before and after the change in length and mass]

Question 5 of 5 NDA MCQ

Which one of the following statements is true for a simple harmonic oscillator?

Detailed Explanation

Correct Answer

โœ“ Option (b) is correct. The force acting on a simple harmonic oscillator is proportional to the displacement from mean position and directed opposite to it.

๐Ÿ“– Governing Law / Core Concept

The motion of a simple harmonic oscillator is governed by Hooke's Law, which states that the restoring force \( F \) is proportional to the displacement \( x \) from equilibrium and acts in the opposite direction: \[ F = -kx \] where \( k \) is the spring constant (N/m).

๐Ÿงช Step-by-Step Breakdown

The essential equations are: \[ a = \frac{F}{m} \] Given \( F = -kx \), substituting yields: \[ a = -\frac{k}{m}x \] Thus, acceleration is also proportional to displacement but opposite in direction, \( a \propto -x \).

๐Ÿ” Option Analysis

- **Option A**: Incorrect, as the force is in the opposite direction of displacement. - **Option C**: Incorrect, acceleration varies with displacement; it is not constant. - **Option D**: Incorrect, the velocity of an oscillator in SHM is periodic.

โšก Mnemonic / Speed-Run

Remember: "SHM is a dance: Force acts back!" (Force opposes displacement).

6. Visual Suggestion

6. Visual Suggestion

A schematic diagram illustrating the position of a mass on a spring, showing the equilibrium position, maximum displacement, and the opposite direction of the restoring force relative to the displacement.

๐Ÿ“– Factual Verification & Reference

๐Ÿ“– Factual Verification & Reference:

Verified against NCERT Class XI Physics, Chapter on Oscillations.

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