Centre of Mass 2 Question 6

7. Two blocks A and B each of mass m, are connected by a massless spring of natural length L and spring constant k. The blocks are initially resting on a smooth horizontal floor with the spring at its natural length, as shown in figure. A third identical block C, also of mass m, moves on the floor with a speed v along the line joining A and B, and collides elastically with A. Then

(1993, 2M)

(a) the kinetic energy of the AB system, at maximum compression of the spring, is zero

(b) the kinetic energy of the AB system, at maximum compression of the spring, is mv2/4

(c) the maximum compression of the spring is v(m/k)

(d) the maximum compression of the spring is vm2k

Analytical & Descriptive Questions

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Answer:

Correct Answer: 7. (b, d)

Solution:

  1. After collision between C and A,C stops while A moves with speed of C i.e. v [in head on elastic collision, two equal masses exchange their velocities]. At maximum compression, A and B will move with same speed v/2 (From conservation of linear momentum).

Let x be the maximum compression in this position.

KE of AB system at maximum compression

=12(2m)v22 or Kmax=mv2/4

From conservation of mechanical energy in two positions shown in above figure

 or 12mv2=14mv2+12kx212kx2=14mv2x=vm2k (Maximum compression) 



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