All Exams Test series for 1 year @ ₹349 only
Question

A thin uniform rod ($X$) of mass $M$ and length $L$ is pivoted at a height $\left(\frac{L}{3}\right)$ as shown in the figure. The rod is allowed to fall from a vertical position and lie horizontally on the table. The angular velocity of this rod when it hits the table top, is __________.
($g = \text{gravitational acceleration}$)

The correct answer is
$\sqrt{\frac{3g}{L}}$

To find the angular velocity of the rod when it hits the table, we can use the principle of conservation of energy. Initially, the rod is in a vertical position with its center of mass at a height of \(\frac{L}{2}\) from the pivot point. When it falls to the horizontal position, the center of mass is at a height of \(\frac{L}{3}\) from the pivot.

Step-by-step Solution:

First, calculate the initial potential energy (PEi) of the rod:

\(PE_i = Mgh\), where \(h = \frac{L}{2}\).

So, \(PE_i = Mg\frac{L}{2}\).

At the horizontal position, calculate the final potential energy (PEf).

The center of mass is at height \(\frac{L}{3}\):

\(PE_f = Mg\frac{L}{3}\).

Using conservation of energy:

Initial Potential Energy = Final Potential Energy + Rotational Kinetic Energy (KErot)

\(Mg\frac{L}{2} = Mg\frac{L}{3} + \frac{1}{2}I\omega^2\)

The moment of inertia \((I)\) of the rod about the pivot is:

\(I = \frac{1}{3}ML^2\) (using the parallel axis theorem).

Substitute \(I\) in the energy conservation equation:

\(Mg\frac{L}{2} - Mg\frac{L}{3} = \frac{1}{2} \times \frac{1}{3}ML^2 \omega^2\)

Simplify and solve for \(\omega\):

\(\frac{MgL}{6} = \frac{1}{6}ML^2\omega^2\)

Cancel \(M\):

\(gL = L^2\omega^2\)

\(\omega^2 = \frac{3g}{L}\)

Therefore, the angular velocity is:

\(\omega = \sqrt{\frac{3g}{L}}\).

Thus, the correct answer is \(\sqrt{\frac{3g}{L}}\).

Was this answer helpful?

Similar Questions

  1. Two masses $m$ and $2m$ are connected by a light string going over a pulley (disc) of mass $30m$ with radius $r=0.1 \text{ m}$. The pulley is mounted in a vertical plane and it is free to rotate about its axis. The $2m$ mass is released from rest and its speed when it has descended through a height of 3.6 m is _________ m/s. (Assume string does not slip and $g = 10 \text{ m/s}^2$)
  2. In the given figure the blocks $A$, $B$ and $C$ weigh 4 kg, 6 kg and 8 kg respectively. The co-efficient of sliding friction between any two surfaces is 0.5. The force $\vec{F}$ required to slide the block $C$ with constant speed is ______ N. (Use $g = 10 \text{ m/s}^2$)

  3. The fifth harmonic of a closed organ pipe is found to be in unison with the first harmonic of an open pipe. The ratio of lengths of closed pipe to that of the open pipe is $5/x$. The value of $x$ is _______.
  4. In a vernier callipers, 50 vernier scale divisions are equal to 48 main scale divisions. If one main scale division $= 0.05 \text{ mm}$, then the least count of the vernier callipers is _______ mm.
  5. A flexible chain of mass $m$ hangs between two fixed points at the same level. The inclination of the chain with the horizontal at the two points of support is $30^\circ$. Considering the equilibrium of each half of the chain, the tension of the chain at the lowest point is _______.
  6. A soap bubble of surface tension $0.04 \text{ N/m}$ is blown to a diameter of $7 \text{ cm}$. If $(15000 - x) \text{ }\mu\text{J}$ of work is done in blowing it further to make its diameter $14 \text{ cm}$, then the value of $x$ is _______.
    ($\pi = 22/7$)
  7. A uniform solid cylinder of length $L$ and radius $R$ has moment of inertia about its axis equal to $I_1$. A small co-centric cylinder of length $L/2$ and radius $R/3$ carved from this cylinder has moment of inertia about its axis equals to $I_2$. The ratio $I_1/I_2$ is _______.
  8. Surface tension of two liquids (having same densities), $T_1$ and $T_2$, are measured using capillary rise method utilizing two tubes with inner radii of $r_1$ and $r_2$ where $r_1 > r_2$. The measured liquid heights in these tubes are $h_1$ and $h_2$ respectively. [Ignore the weight of the liquid about the lowest point of meniscus]. The heights $h_1$ and $h_2$ and surfaces tensions $T_1$ and $T_2$ satisfy the relation :
  9. Two cars $A$ and $B$ each of mass $10^3\text{ kg}$ are moving on parallel tracks separated by a distance of $10\text{ m}$, in same direction with speeds $72\text{ km/h}$ and $36\text{ km/h}$. The magnitude of angular momentum of car $A$ with respect to car $B$ is __________ $\text{J.s}$.
  10. A spherical body of radius $r$ and density $\sigma$ falls freely through a viscous liquid having density $\rho$ and viscosity $\eta$ and attains a terminal velocity $v_0$. Estimated maximum error in the quantity $\eta$ is : (Ignore errors associated with $\sigma, \rho$ and $g$, gravitational acceleration)

Important Questions from Mechanics

  1. Two masses $m$ and $2m$ are connected by a light string going over a pulley (disc) of mass $30m$ with radius $r=0.1 \text{ m}$. The pulley is mounted in a vertical plane and it is free to rotate about its axis. The $2m$ mass is released from rest and its speed when it has descended through a height of 3.6 m is _________ m/s. (Assume string does not slip and $g = 10 \text{ m/s}^2$)
  2. In the given figure the blocks $A$, $B$ and $C$ weigh 4 kg, 6 kg and 8 kg respectively. The co-efficient of sliding friction between any two surfaces is 0.5. The force $\vec{F}$ required to slide the block $C$ with constant speed is ______ N. (Use $g = 10 \text{ m/s}^2$)

  3. The fifth harmonic of a closed organ pipe is found to be in unison with the first harmonic of an open pipe. The ratio of lengths of closed pipe to that of the open pipe is $5/x$. The value of $x$ is _______.
  4. In a vernier callipers, 50 vernier scale divisions are equal to 48 main scale divisions. If one main scale division $= 0.05 \text{ mm}$, then the least count of the vernier callipers is _______ mm.
  5. A flexible chain of mass $m$ hangs between two fixed points at the same level. The inclination of the chain with the horizontal at the two points of support is $30^\circ$. Considering the equilibrium of each half of the chain, the tension of the chain at the lowest point is _______.
Need Expert Advice?
More Questions from JEE Main

Start Your Preparation with Prepp Mobile App

Download the app from Google Play & App Store
Download the app from Google Play & App Store
Prepp Mobile App