#### Problem 10E

10. A particle moves with position function

$$s=t^{4}-4 t^{3}-20 t^{2}+20 t \quad t \geqslant 0$$

(a) At what time does the particle have a velocity of $20 \mathrm{~m} / \mathrm{s} ?$

(b) At what time is the acceleration $0 ?$ What is the significance of this value of $t$ ?