The accompanying figure shows the velocity v = f(t) of a particle moving on a coordinate line. (a) When does the particle move forward? move backward? speed up? slow down? (b) When is the particle's acceleration positive? negative? zero? (c) When does the particle move at its greatest speed? (d) When does the particle stand still for more than an instant? 5- v = f(t) 2 4 5 6 7 8 9 10 t (sec) ✓