MCQMediumJEE 2024Newton's Second Law & Force

JEE Physics 2024 Question with Solution

All surfaces shown in the figure are frictionless, and the pulleys and the string are light. The acceleration of the block of mass 2kg2 \, \text{kg} is:

  • A

    gg

  • B

    g/3g/3

  • C

    2g/32g/3

  • D

    g/4g/4

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: All surfaces are frictionless, and the pulleys and string are light. The required quantity is the acceleration of the block of mass 2kg2 \, \text{kg}.

Find: Acceleration of the 2kg2 \, \text{kg} block.

The solution states that the correct option is B and concludes that the acceleration is g/3g/3.

From the extracted working:

2a=gsin(30)=g22a = g \sin(30^\circ) = \frac{g}{2}

and for the other mass,

4a=2ga=g34a = 2g \Rightarrow a = \frac{g}{3}

Thus, the acceleration of the block of mass 2kg2 \, \text{kg} is g/3g/3.

Therefore, the correct option is B.

Working

Given: The system is frictionless and uses light pulleys and string.

Find: The acceleration of the 2kg2 \, \text{kg} block.

The first solution passage says that for the 2kg2 \, \text{kg} block, using Newton's second law,

T=2aT = 2a

It then mentions force balance relations in the pulley system and concludes that the acceleration should be g/3g/3.

The second approach explicitly provides:

2a=gsin(30)=g22a = g \sin(30^\circ) = \frac{g}{2}

and then states for the 4kg4 \, \text{kg} mass,

4a=2ga=g34a = 2g \Rightarrow a = \frac{g}{3}

Although the intermediate equations in the solution are not fully consistent, the solution clearly identifies option B and concludes the acceleration as g/3g/3.

Therefore, the acceleration is g/3g/3 and the correct option is B.

Common mistakes

  • Using the weight of the 2kg2 \, \text{kg} block directly as the net accelerating force is incorrect because the motion depends on the full pulley constraint and tension relations. Write Newton's second law for each connected part of the system instead.

  • Assuming the tension is different at different points of the same light string is wrong for an ideal massless string over frictionless pulleys. Use the same tension throughout a continuous string unless the setup clearly shows otherwise.

  • Ignoring geometric or constraint relations between accelerations can lead to a wrong answer such as gg or g/2g/2. First identify how the connected blocks and pulleys relate the accelerations before solving the force equations.

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