Answer
(a) object speeds up or slows down.
(b) object moves in a parabolic path.
(c) object is in circular motion.
Work Step by Step
(a) When acceleration is parallel to initial velocity, the magnitude of the object's velocity either increases or decreases.
\begin{align*}
\vec v=\vec u+\vec a\,t
\end{align*} Its direction remains the same.
(b) When the acceleration is perpendicular to initial velocity, the component of the object's velocity along the direction of the acceleration increases consistently. The object moves in a parabolic path given by coordinates -
\begin{align*}
s_x&=u_xt\\
s_y&=\frac{1}{2}a_yt^2
\end{align*}(c) For acceleration always perpendicular to instantaneous velocity, the object moves in a circular path.
