A projectile enters a resisting medium at x = 0 with an initial velocity vo = 900 ft/s and travels 4 in. before coming to rest. Assuming that the velocity of the projectile is defined by the relation v = vo - kx, where v is expressed in ft/s and x is in feet, determine (a) the initial Fig. P11.16 acceleration of the projectile, (b) the time required for the projectile to penetrate 3.9 in. into the resisting medium.