Subject : Physics - Example Questions - Torque and Equilibrium:

Question 1: Lever Balance

A uniform meter stick of mass M is placed on a fulcrum at the 60 cm mark. A 2 kg mass is hung at the 20 cm mark on one side, and a 3 kg mass is hung at the 90 cm mark on the other side. Determine the mass M of the meter stick. Assume the meter stick is of negligible mass.



Question 2: Hanging Rod Equilibrium

Consider a uniform rod of length L and mass M hanging horizontally from a ceiling. Two masses, m1 and m2, are attached at the ends of the rod. If m1 > m2, determine the point along the rod where it should be supported by a string attached to the ceiling for the system to be in equilibrium.



Kinetic Energy of Rotation:

Question 3: Rolling Hoop vs. Solid Disk

A solid disk and a hoop, both of the same mass and radius, are released from rest at the top of an inclined plane. Neglecting friction, compare their linear speeds at the bottom of the incline. Justify your answer using the conservation of energy.



Question 4: Rotating Rod

A thin rod of length L and mass M is rotating about its end with an angular velocity ω. Determine the kinetic energy of rotation of the rod.



Velocity on an Inclined Plane using Moment of Inertia:

Question 5: Rolling Down an Incline

A solid sphere of radius R and mass M starts from rest and rolls without slipping down an inclined plane of height h and angle θ. Calculate the velocity of the center of mass of the sphere when it reaches the bottom of the incline. Use the moment of inertia for a solid sphere.



Answer :

Final Answer:

Answer 1: The mass [tex]\(M\)[/tex] of the meter stick is [tex]\(6 \, \text{kg}\)[/tex].

Answer 2: To achieve equilibrium when [tex]\(m_1 > m_2\)[/tex], the rod should be supported at a point [tex]\(x\)[/tex] where [tex]\(x = \frac{m_2}{m_1 + m_2} \times L\)[/tex].

Answer 3: The linear speeds of the solid disk and hoop at the bottom of the incline are equal due to the conservation of energy.

Answer 4: The kinetic energy of rotation of the rotating rod is [tex]\( \frac{1}{3} M L^2 \omega^2 \).[/tex]

Answer 5: The velocity of the center of mass of the sphere when it reaches the bottom of the incline is [tex]\( \sqrt{\frac{2gh}{1 + \frac{2}{5}}}\)[/tex].

Explanation:

Answer 1: To find [tex]\(M\)[/tex], apply the principle of torque equilibrium: [tex]\(\sum \tau = 0\)[/tex]. The torques due to the masses must balance. The torque from the 2 kg mass [tex](\(20 \, \text{cm}\))[/tex] is [tex]\(2 \times 9.8 \times 0.2 \, \text{m} = 3.92 \[/tex], [tex]\text{Nm}\)[/tex], and from the 3 kg mass [tex](\(90 \, \text{cm}\))[/tex] is [tex]\(3 \times 9.8 \times 0.3 \, \text{m} = 8.82 \, \text{Nm}\)[/tex]. For equilibrium, [tex]\(3.92 + M \times 9.8 \times 0.6 = 8.82\), yielding \(M = 6 \, \text{kg}\)[/tex].

Answer 2: For equilibrium, the torques due to [tex]\(m_1\) and \(m_2\)[/tex] must balance. Using [tex]\(\sum \tau = 0\), \(m_1 \times x = m_2 \times (L - x)\)[/tex], solving for [tex]\(x\)[/tex] gives [tex]\(x = \frac{m_2}{m_1 + m_2} \times L\).\\[/tex]

Answer 3: Applying conservation of energy, the potential energy at the top is converted into kinetic energy at the bottom. Equating the energies of the hoop and disk, the linear speeds are the same.

Answer 4: The kinetic energy of rotation for a thin rod rotating about its end is [tex]\( \frac{1}{3} M L^2 \omega^2 \).[/tex]

Answer 5: Using energy conservation, the potential energy at the top converts to translational and rotational kinetic energy at the bottom, yielding the velocity formula [tex]\( \sqrt{\frac{2gh}{1 + \frac{2}{5}}}\)[/tex].