本题目来源于试卷: 2012美国US F=MA物理竞赛，类别为 美国F=MA物理竞赛
[单选题]
A mass hangs from the ceiling of a box f,xkmzh9ly -t *v t(b4by an ideal springyqm3 +nv q*70gq6pdp qt/a7dh. With the box held fixed, the mass is given an initial velocity and oscillates with pu qqpndq30*t7a /dgm 7v+y6phq rely vertical motion. When the mass reaches the lowest point of its motion, the box is released and allowed to fall. To an observer inside the box, which of the following quantities does not change when the box is released? Ignore air resistance.
A. The amplitude of the oscillation
B. The period of the oscillation
C. The maximum speed reached by the mass
D. The height at which the mass reaches its maximum speed
E. The maximum height reached by the mass
参考答案: B
本题详细解析:
The period of a simple harmonic oscillator (:v /2ezn)pri r)sc5 ltmass on a spring) is $T = 2\pi\sqrt{m/k}$. This period depends only on the mass $m$ and the spring constant $k$, neither of which changes.
When the box is released, it enters free fall. In the accelerating frame of the box, the force of gravity $mg$ is exactly canceled by the upward pseudo-force $-ma = -m(-g) = +mg$. The *only* force on the mass (in this frame) is the spring force.
This changes the equilibrium position (from $k\Delta y = mg$ to $k\Delta y = 0$), which in turn changes the amplitude, maximum speed, and equilibrium height. But the period $T$ remains the same.
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