8   3. A stone of mass 0.5 kg is projected vertically upwards with a speed U m s–1 from a point A. The point A is 2.5 m above horizontal ground. The speed of the stone as it hits the ground is 25 m s–1 The motion of the stone from the instant it is projected from A until the instant it hits the ground is modelled as that of a particle moving freely under gravity. (a) Use the model and the principle of conservation of mechanical energy to find the value of U. (4) In reality, the stone will be subject to air resistance as it moves from A to the ground. (b) State how this would affect your answer to part (a). (1) The ground is soft and the stone sinks a vertical distance d cm into the ground. The resistive force exerted on the stone by the ground is modelled as a constant force of magnitude 2000 N and the stone is modelled as a particle. (c) Use the model and the work-energy principle to find the value of d, giving your answer to 3 significant figures. 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