# Suppose the Rod in the Previous Problem Has a Mass of 1 Kg Distributed Uniformly Over Its Length. (A) Find the Initial Angular Acceleration of the Rod. - Physics

Sum

Suppose the rod in the previous problem has a mass of 1 kg distributed uniformly over its length.

(a) Find the initial angular acceleration of the rod.

(b) Find the tension in the supports to the blocks of mass 2 kg and 5 kg.

#### Solution

Total moment of inertia of the system about the axis of rotation,

$I_{net} = \left( m_1 r_1^2 + m_2 r_2^2 + \frac{m l^2}{12} \right)$

m and l are the mass and length of the rod, respectively.

$\tau_{net} = F_1 r_1 - F_2 r_2$

$\text{Also, }\tau_{net} = I_{net} \times \alpha$

On equating the value of $\tau_{net}$ and putting the value of l_("net"), we get

$F_1 r_1 - F_2 r_2 = \left( m_1 r_1^2 + m_2 r_2^2 + \frac{m l^2}{12} \right) \times \alpha$

$\left( - 2 \times 10 \times 0 . 5 \right) + \left( 5 \times 10 \times 0 . 5 \right) = \left[ 5 \left( \frac{1}{2} \right)^2 + 2 \left( \frac{1}{2} \right)^2 + \frac{\left( 1 \right)^2}{12} \right] \alpha$

$\Rightarrow 15 = \left( 1 . 75 + 0 . 084 \right) \alpha$

$\Rightarrow \alpha = \frac{1500}{\left( 175 + 8 . 4 \right)} = \frac{1500}{183 . 4}$

$= 8 . 1\text{ rad/s}^2 ........\left( g = 10 \right)$

$= 8 . 01\text{ rad/s}^2 ......\left(\text{if }g = 9 . 8 \right)$

(b) From the free body diagram of the block of mass 2 kg,

$T_1 - m_1 g = m_1 a$

$\Rightarrow T_1 = 2 \left( a + g \right)$

$= 2\left( \alpha r + g \right)........ \left(\text{using, }a = \alpha r \right)$

$= 2\left( 8 \times 0 . 5 + 9 . 8 \right)$

$\Rightarrow T_1 = 27 . 6 N$

From the free body diagram of the block of mass 5 kg,

$m_2 g - T_2 = m_2 a$

$\Rightarrow T_2 = m_2 \left( g - a \right)$

$= 5 \left( g - a \right) = 5 \left( 9 . 8 - 8 \times 0 . 5 \right)............\left( a = \alpha r \right)$

$= 5 \times 5 . 8 = 29 N$

Concept: Values of Moments of Inertia for Simple Geometrical Objects (No Derivation)
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#### APPEARS IN

HC Verma Class 11, 12 Concepts of Physics 1
Chapter 10 Rotational Mechanics
Q 32 | Page 197