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Laws of Motion question

2007 · Shift 2 · Q19
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Laws of Motion question

2007 · Shift 2 · Q19

JEE AdvancedPhysicsLaws of MotionMCQ+3 / −1
STATEMENT 1 A cloth covers a table. Some dishes are kept on it. The cloth can be pulled out without dislodging the dishes from the table. STATEMENT 2 For every action there is an equal and opposite reaction.
  1. A
    Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1.
  2. B
    Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1s.
  3. C
    Statement-1 is True, Statement-2 is False.
  4. D
    Statement-1 is False, Statement-2 is True.
View written solutionFree

Correct answer: B

Analysis of Statement 1

  1. The Statement: "A cloth covers a table. Some dishes are kept on it. The cloth can be pulled out without dislodging the dishes from the table."
  2. Physical Principle: This statement describes a well-known physics demonstration often called the "tablecloth trick". The phenomenon is explained by the principle of inertia, which is described by Newton's First Law of Motion. Inertia is the tendency of an object to resist changes in its state of motion.
  3. Explanation: The dishes are initially at rest. Due to their mass, they possess inertia and tend to remain at rest. When the cloth is pulled from underneath them, a frictional force acts on the dishes for the duration of the pull.
  4. Impulse and Momentum: The impulse (JJJ) given to the dishes is the product of the frictional force (FfF_fFf​) and the time interval ("Deltat""Delta t""Deltat") for which it acts: J=FfΔtJ = F_f \Delta tJ=Ff​Δt. According to the impulse-momentum theorem, this impulse equals the change in momentum ("Deltap""Delta p""Deltap") of the dishes: "Deltap=J""Delta p = J""Deltap=J".
  5. Effect of a Quick Pull: If the cloth is pulled out very quickly (a sharp jerk), the time interval "Deltat""Delta t""Deltat" is very small. Consequently, the impulse transferred to the dishes is also very small. A small impulse results in a small change in momentum, and therefore the dishes acquire only a very small velocity. They move a negligible horizontal distance and remain on the table.
  6. Conclusion: The statement is a correct description of a real physical phenomenon. Thus, Statement 1 is True.

Analysis of Statement 2

  1. The Statement: "For every action there is an equal and opposite reaction."
  2. Physical Principle: This is a direct statement of Newton's Third Law of Motion.
  3. Conclusion: This is one of the fundamental laws of classical mechanics and is universally accepted as true within its domain of applicability. Thus, Statement 2 is True.

Relationship between Statement 1 and Statement 2

  1. Correct Explanation for Statement 1: As established above, the primary reason the dishes are not dislodged is their inertia (Newton's First Law) and the fact that a small impulse is imparted to them due to the short duration of the applied force (related to Newton's Second Law, F=maF = maF=ma or F=dpdtF = \frac{dp}{dt}F=dtdp​).
  2. Relevance of Statement 2: Newton's Third Law (Statement 2) states that the frictional force the cloth exerts on the dishes is equal and opposite to the frictional force the dishes exert on the cloth. While this is true for the forces involved in the interaction, it does not explain the outcome of the action on the dishes. The motion (or lack thereof) of the dishes is determined by the net force acting on the dishes and the time for which it acts, not by the existence of a reaction force on the cloth.
  3. Conclusion: Although both statements are true physical principles, Newton's Third Law (Statement 2) is not the correct explanation for the phenomenon described in Statement 1. The correct explanation is based on Newton's First and Second Laws (inertia and impulse).

Final Decision

  • Statement 1 is True.
  • Statement 2 is True.
  • Statement 2 is NOT a correct explanation for Statement 1.

This corresponds to option B.

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