Machinedesign 2108 Stop Gap 0 0

Fun With Fundamentals: Problem 200

Nov. 1, 2000
You need a good set of brakes to go the distance, as this month's problem by Jerry Reckelhoff of Cincinnati demonstrates

Problem 200 — You need a good set of brakes to go the distance, as this month’s problem by Jerry Reckelhoff of Cincinnati demonstrates.

It was a sunny Sunday afternoon, and Finagel J. Wurme was enjoying an exhilarating ride in the country on his motorcycle. The picturesque lane he traveled at 60 mph had a downhill gradient of 5 deg. and crossed a babbling brook. Suddenly a sign, “Bridge Out 500 ft” jarred him out of his reverie. Wurme immediately hit the brakes, generating a constant braking force of 250 lb. If Wurme and motorcycle weigh a total of 750 lb, does Wurme end up in the brook? How many feet does it take him to come to a complete stop?

Send your answer to:

Fun With Fundamentals
1100 Superior Ave.
Cleveland, OH 44114-2543

Deadline is January 10. Good luck!

Technical consultant, Jack Couillard, Menasha, Wis.

Solution to last month’s problem 199 — You are no Doubting Thomas if you answered 10 ft. Here’s how McGroundbound saw the light:

For the main-rotor blade to appear to be still, its tangential velocity must equal the helicopter’s linear velocity, but be in the opposite direction. Let:

v = Helicopter’s linear velocity, given as 150 knots or 253 fps
ω = Tangential velocity of main-rotor blade, radians/sec
= Radius of main-rotor blade that appears to be standing still, ft

From physics,

McGroundbound heard eight “slaps” per second from a two-bladed main rotor.

That means the rotor was turning at 4 rps. Plug these values into (1):

Thus the part of the rotor “standing still” is on the retreating blade at a point 10 ft from the rotor’s hub. Skylark helped himself to the dessert while McGroundbound pondered these mysteries of perception.

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!