Phase space diagrams are useful tools in analyzing all kinds of dynamical problems. They are especially useful in studying the changes in motion as initial position and momentum are changed. Here we consider some simple dynamical systems in one-dimension. For such systems, phase space is a plane in which position is plotted along horizontal axis and momentum is plotted along vertical axis. The phase space diagram is x(t) vs. p(t) curve in this plane. The arrow on the curve indicates the time flow. For example, the phase space diagram for a particle moving with constant velocity is a straight line as shown in the figure. We use the sign convention in which position or momentum upwards (or to right) is positive and downwards (or to left) is negative.
The phase space diagram for simple harmonic motion is a circle centered at the origin. In the figure, the two circles represent the same oscillator but for different initial conditions, and E1 and E2 are the total mechanical energies respectively. Then
(A)E1=2E2
(B)E1=2E2
(C)E1=4E2✓ Correct
(D)E1=16E2
Answer: (C)
Solution
For simple harmonic motion, the phase space trajectory is an ellipse (or circle with appropriate scaling). The energy is proportional to the square of the amplitude.
From the figure, the radius of the larger circle appears to be twice that of the smaller circle. Since energy E∝A2 (where A is amplitude):
E2E1=(A2A1)2=(A22A2)2=4
Therefore, E1=4E2.
JEE Advanced · 2011 · 10 Apr 2011 · Paper 1 · Single correct option