역학 문제 좀 풀어주세요

Two blocks, both of mass m, are fastened to a spring as shown in the figure. The length of the unforced spring is L, the spring constant is k. We neglect the mass of the spring. The system is attached to the ceiling of the laboratory with a string. The acceleration of gravity is g. Initially, the system hangs at rest. At time t=0 the string is cut, and the system falls freely in the gravitational field. We neglect air resistance.

(1) Determine the position of the center of mass (CM) at times t>0. The position should be given as the distance yCM from the CM position at t=0.

answer:  yCM = ½gt2

(2) Determine the acceleration a1 of the top mass (1) and the acceleration a2 of the bottom mass(2) just after the string has been cut.

answer:  a(1)=2g
             a(2)=0

We mow refer the motion for t>0 to a reference frame that has its origin in the CM. Denote in this frame the position of the lower mass (2) by z.

(3) Determine z as a function of t (consider only values of t such that (2) has not hit the floor).

answer:   x=L/2+m*g/2/k*cos(sqrt(2*k/m)*t)


그림 있는 건 첨부자료에 있습니다.
특히 (3)번은 어떻게 해야 하는지 잘 모르겠습니다.
아시는 분 도움 좀 주시기 바랍니다.
* 안시원님에 의해서 게시물 복사되었습니다 (2004-09-03 16:22)