4 A ball is released from rest at a height of 15 metres above ground level.
- Find the speed of the ball when it hits the ground, assuming that no air resistance acts on the ball.
- In fact, air resistance does act on the ball. Assume that the air resistance force has a constant magnitude of 0.9 newtons. The ball has a mass of 0.5 kg .
- Draw a diagram to show the forces acting on the ball, including the magnitudes of the forces acting.
- Show that the acceleration of the ball is \(8 \mathrm {~m} \mathrm {~s} ^ { - 2 }\).
- Find the speed at which the ball hits the ground.
- Explain why the assumption that the air resistance force is constant may not be valid.