Look at the wave direction first, and then determine the particle motion direction. The phase of the particle at the highest point is π/2, and the phase at the lowest point is -3π/2. In other positions, it should be judged according to the direction of movement. For example, on the coordinate axis, the phase of downward motion is-π.

Initial phase

Since it is college physics, we must use the rotation vector method. When t = 0, the particle is moving forward through the midpoint of the maximum value. The corresponding vector is 60 degrees between the fourth quadrant and the X axis, and the corresponding phase is 5/3π or-1/3π. The negative angle is the angle formed by clockwise rotation of the positive direction of the X axis.

Sine voltage source U=Um sin(wt+φ), where wt+φ is called the phase angle of sine, and when t=0, φ is called the initial phase or initial phase angle [1]. φ is equivalent to the angle between the zero-crossing point and the coordinate 0 point.