Question

Karl starts the engine on his small private airplane. The engine drives a propeller with a radius of 9 feet and its centerline 12.5 feet above the ground. At idle, the propeller rotates at a constant speed of approximately 750 revolutions per minute. The height of one propeller tip as a function of time is given by h = 12.5 + 9 sin(750t), where h is the height in feet and t is the time in minutes. Find h when t = 3.5 minutes.

Answer 1

a=21.1

Explanation:

Answer 2

• 3.68 feet using given equation (wrong)
• 12.5 feet using correct equation

Explanation:

You can use the given (incorrect) equation and fill in the value of t to find h:

h = 12.5 +9sin(750(3.5)) = 3.68 . . . . feet

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Or, you can use the correct equation, or just your knowledge of revolutions:

h = 12.5 +9sin(750(2π·3.5)) = 12.5 . . . . feet

in 3.5 minutes at 750 revolutions per minute, the propeller makes 2625 full revolutions, so is back where it started — at 12.5 feet above the ground.