It is because it goes off on a tangent. The direction of motion is perpendicular to the line of the pendulum at all times.
For the pendulum just review L18 video 3. For a) you should use the conservation of mechanical energy (mv(bot)^2)/2 = (mv0^2)/2 + mgh where h is the distance of the ball from the ground calculated as h = L*(1-cos(theta)). For b) put together y component of gravitational force ...
5 Physics dollars.