2/143 The slider P can be moved inward by means of the string S, while the slotted arm rotates about point 0. The angular position of the arm is given

by \theta=0.8 t-\frac{t^{2}}{20}, \text { where } \theta \text { is in radians and } t \text { is in } seconds. The slider is at r = 1.6 m when t = 0 and thereafter is drawn inward at the constant rate of 0.2 m/s. Determine the magnitude and direction(expressed by the angle a relative to the x-axis)of the velocity and acceleration of the slider when t = 4 s.