15-11. During operation the jack hammer strikes the concrete surface with a force which is indicated in the graph. To achieve this the 2-kg spike S is fired into the surface at 90 m>s. Determine the speed of the spike just after rebounding.
15-14. A tankcar has a mass of 20 Mg and is freely rolling to the right with a speed of 0.75 m> s. If it strikes the barrier, determine the horizontal impulse needed to stop the car if the spring in the bumper B has a stiffness
(a) k S 8 (bumper is rigid), and
(b) k = 15 kN> m.