Question

plot a velocity-time graph showing +a (positive acceleration) and -a (negative acceleration).

Answer 1

Take 1st equation of linear motion.That is v = u+at

                                                                    at = v-u

                                                                     a = (v-u)/t

                                                  acceleration = rate of change of velocity

That means,acceleration is given by the slope of velocity vs time (v-t) graph.

If there is a  

positive acceleration⇒ positive slope in v-t graph⇒

                                                           make acute angle with positive x axis

negative acceleration⇒negative slope in v-t graph⇒

                                                          make obtuse angle with positive x axis