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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Centre of Mass, Linear momentum, Collision

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Centre of Mass, Linear momentum, Collision

Evaluate the following two statements concerning a system of...

Evaluate the following two statements concerning a system of particles:
(A) The linear momentum of the system is zero.
(B) The kinetic energy of the system is zero.

Options

  1. AB implies A, but A does not imply B
  2. BA does not imply B, and B does not imply A
  3. CA implies B, but B does not imply A
  4. DA implies B, and B implies A

Correct answer

A. B implies A, but A does not imply B

Step-by-step solution

The kinetic energy of a system of particles is given by K=∑12mivi2K = \sum \dfrac{1}{2} m_i v_i^2.

If the kinetic energy of the system is zero, i.e., K=0K = 0, then vi=0v_i = 0 for all particles since the kinetic energy of individual particles cannot be negative.

If all particles are at rest, the linear momentum of the system P⃗=∑miv⃗i\vec{P} = \sum m_i \vec{v}_i must also be zero. Thus, statement B implies statement A.

The linear momentum of a system is given by P⃗=∑miv⃗i\vec{P} = \sum m_i \vec{v}_i.

If the linear momentum of the system is zero, it is possible that individual particles are moving such that their vector sum of momenta is zero (for example, two particles of equal mass moving with equal speeds in opposite directions). In this case, the kinetic energy of the system will be non-zero.

Thus, statement A does not imply statement B.
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