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Distance Traversed by Centre of Mass of Pulley System in Given Time

Two blocks of masses 2 kg2\text{ kg} and 1 kg1\text{ kg} respectively, are tied to the ends of a string which passes over a light frictionless pulley as shown in the figure below. The masses are held at rest at the same horizontal level and then released. The distance traversed by the centre of mass in 2 s2\text{ s} is ______ m\text{m}. (Take g=10 m/s2g = 10\text{ m/s}^2)

Question Diagram 1

Options

A

3.333.33

B

3.123.12

C

2.222.22

Correct
D

1.421.42

Step-by-Step Solution

To find the distance traversed by the centre of mass of the system in t=2 st = 2\text{ s}, we first determine the acceleration of each block.

Let the masses be m1=2 kgm_1 = 2\text{ kg} and m2=1 kgm_2 = 1\text{ kg}.

The common acceleration aa of the system of blocks connected over a light, frictionless pulley is given by: a=(m1m2m1+m2)ga = \left(\frac{m_1 - m_2}{m_1 + m_2}\right) g

Substitute the given values (g=10 m/s2g = 10\text{ m/s}^2): a=(212+1)×10=103 m/s2a = \left(\frac{2 - 1}{2 + 1}\right) \times 10 = \frac{10}{3}\text{ m/s}^2

Here, mass m1=2 kgm_1 = 2\text{ kg} moves vertically downwards with acceleration aa, while mass m2=1 kgm_2 = 1\text{ kg} moves vertically upwards with acceleration aa.

Taking the vertically downward direction as positive, the acceleration of the individual masses can be represented vectorially as: a1=+103j^ m/s2\vec{a}_1 = +\frac{10}{3}\hat{j}\text{ m/s}^2 a2=103j^ m/s2\vec{a}_2 = -\frac{10}{3}\hat{j}\text{ m/s}^2

The acceleration of the centre of mass (acm\vec{a}_{\text{cm}}) is given by: acm=m1a1+m2a2m1+m2\vec{a}_{\text{cm}} = \frac{m_1 \vec{a}_1 + m_2 \vec{a}_2}{m_1 + m_2}

Substituting the values: acm=2(+103j^)+1(103j^)2+1=203j^103j^3=109j^ m/s2\vec{a}_{\text{cm}} = \frac{2 \left(+\frac{10}{3}\hat{j}\right) + 1 \left(-\frac{10}{3}\hat{j}\right)}{2 + 1} = \frac{\frac{20}{3}\hat{j} - \frac{10}{3}\hat{j}}{3} = \frac{10}{9}\hat{j}\text{ m/s}^2

The magnitude of the acceleration of the centre of mass is: acm=109 m/s2a_{\text{cm}} = \frac{10}{9}\text{ m/s}^2

Since the system starts from rest (ucm=0u_{\text{cm}} = 0), the distance scms_{\text{cm}} traversed by the centre of mass in time t=2 st = 2\text{ s} is: scm=ucmt+12acmt2s_{\text{cm}} = u_{\text{cm}} t + \frac{1}{2} a_{\text{cm}} t^2 scm=0+12×(109)×(2)2=209 m2.22 ms_{\text{cm}} = 0 + \frac{1}{2} \times \left(\frac{10}{9}\right) \times (2)^2 = \frac{20}{9}\text{ m} \approx 2.22\text{ m}

Thus, the correct option is C.

Distance Traversed by Centre of Mass of Pulley System in Given Time | Physics PYQ Solution - JEE Challenger