In figure (a) T = 2 s; A = 3 cm At t = 0, OP makes angle $\frac{\pi}{2}$ with x-axis, i.e. $=\frac{\pi}{2}$ radian. While moving clockwise, here $\phi = +\frac{\pi}{2}$. Thus the x-projection of OP at time t will give us the equation of simple harmonic motion, given by $x = A \cos\left( \frac{2\pi t}{T} + \pi \right)$ $= 3 \cos\left( \frac{2\phi t}{2} + \frac{\pi}{2} \right)$ or $x = -3 \sin(\pi t)$ (where x is in cm) In figure (b) T = 4 s ; A = 2 m At t = 0, OP makes an angle p with the positive direction of x-axis, i.e. $\phi = \pi$. While moving anticlockwise, here $\phi = +\pi$. Thus the x-projection of OP at time t will give us the equation of simple harmonic motion given by$x = A \cos\left( \frac{2\pi t}{T} + \phi \right)$ $= 2 \cos\left( \frac{2\pi t}{4} + \pi \right) = -2 \cos\left( \frac{\pi}{2} t \right)$(where x is in m)