A particle moves according to a law of motion s = f(t), t 0, where t is measured in seconds and s in feet. (If an answer does not exist, enter DNE.) f(t) =t3 9t2+24t (a) Find the velocity (in ft/s) at time t. v(t)=32 18t+24 - ft/s (b) What is the velocity (in ft/s) after 1 second? v(1) = 9 ft/s (c) When is the particle at rest? (Enter your answers as a comma-separated list.) t = 2, 4 (d) When is the particle moving in the positive direction? (Enter your answer using interval notation.) (-00,2) U (4,00)