4. (a) A spring with Hooke's constant 2.1 - runs horizontally and is attached to a block of mass 8.4 kg which can slide on a frictionless surface. Initially the block has speed of zero and the spring is compressed 4.8 m from its rest length. What is the speed of the block at the moment the spring has returned to its rest length? Correct quantity: 2.4m (b) A sphere of mass m - 2.0kg and radius 0,40m rolls (without rolling-friction and without slipping) down an incline a vertical distance h - 3.5 m. Given that the rotational inertia of the sphere is / - mr , what is the rotational velocity w of the sphere when it reaches the bottom of the incline? (Recall that the linear speed v = rw, linear kinetic energy Ky = mus, rotational kinetic energy KR - Iw and gravitational potential energy P = might Correct quantity: 7.0m.