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? Can you water-ski being towed by an Olympic rowboat (thin rowboat with an eight man team)?
A light person weighing about 90 pounds can. The eight team members will have to give it every thing they have. The required speed is about 12.5 mph.
? Can you water-ski at a walking pace? If yes, how
light does the skier have to be?
A large surface is required. A surface of about 3 m² is required for someone weighing 220 pounds.
? In our imaginary lake, there is no low-viscosity water. Instead there is
a liquid that is 100 times thicker such as motor oil. How does this influence
the results of our thought experiment?
You can do everything at a much slower speed.
? Can you water-ski when the skis aren't curved in the front?
Yes but with the skis at a sharper angle in the water.
? Why do tires with no tread aquaplane quicker than with
tires that have a tread?
When the tread is deep, the water pressure builds up much more slowly because the water can flow more quickly to the side.
?Did you find a mistake in the Coulomb test results on the
friction coefficients on page 9?
Aha, you may say, the friction coefficient for "Metal/metal, dry" should be 0.8 and not 0.18. A measuring error or printing error?
? How many people would it take to move the portrayed Egyptian statue
if we could use a "super antifriction coating," where f = 0.01?
Assuming that the weight of the statue was 500,000 N (50 tons), then, given f = 0.01, we would need a corresponding tensile force of of 5000 N which would take 10 to 20 men!
The Egyptians would have been extremely grateful to have a "super antifriction coating" since the objects that they hauled around could weigh more than 1,000 tons!
? Why, when you stomp on the brakes, is the brake path for a car with an anti-lock
brake system sometimes a bit longer on a dry street than when there is optimum slip?
Slip increases friction and decreases the brake path; see page 28.
? Does "smooth" mean slippery? If yes, why? What is the
friction factor for a very smooth plastic surface when gripped by the tips of our fingers?
Especially when they are slightly moist?
Try it with a CD.
At what angle does the CD slip off your finger tips? With a bit of patience, you can achieve surprising angles. What is the calculated friction factor?
Hard to believe?
Fig. 5.01
CD on finger tips
a > 80º, f = tan a > 5
Take three SuperBalls*. What does the friction factor have to be so that they can be held like this?
Fig. 5.02 Fig. 5.03
SuperBalls with an astoundingly high friction
In the picture on the left, the middle ball is "glued" below the two top ball.
*You can get SuperBalls in gum dispensers at supermarkets or in toy stores.
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