13. A Dutch windmill has blades that are 20 m in length, and the centre of their circular motion is a point 23 m above the ground. The blades have a frequency of 4 revolutions per minute when in operation. a) Use a sinusoidal function to model the height above the ground of the tip of one blade as a function of time. Assume that the blade starts from the highest point. (3 marks, T) b) How far off the ground is the tip of the blade at 10 s? (1 mark, A) c) Find the instantaneous rate of change of the height at 10 seconds. Explain what does it tell you about the motion of the windmill at this time. (3 marks, A; 1 mark, C)