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Review Example: Friction

Review Example: Friction. Assuming that Superman weighs 200 lb , the boat weighs 10 tons, and the coefficients of friction between the boat and dock are µ s = 0.5 and µ k = 0.25 find: The minimum coefficient of static friction between Superman’s feet and the dock to move the boat at all.

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Review Example: Friction

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  1. Review Example: Friction • Assuming that Superman weighs 200 lb, the boat weighs 10 tons, and the coefficients of friction between the boat and dock are µs = 0.5 and µk = 0.25 find: • The minimum coefficient of static friction between Superman’s feet and the dock to move the boat at all. • The minimum coefficient of static friction to accelerate the boat at 10 ft/s2.

  2. Superman’s Sticky Feet • To get the boat to just budge, Superman must provide a force equal to the weight of the boat times its coefficient of static friction • For Superman to stay stationary, by Newton’s 3rd law, the ground must provide an equal and opposite force to Superman’s pull • Static case: • Dynamic case: • Force on boat: • Friction: • Clearly Superman is much heavier than we think or has some really sticky feet!

  3. Example: Snakes on a Plane • Is friction good or bad? How does it affect you in your daily life? • Snakes are masters at manipulating friction to move • They (and you) can’t move without friction • They also can’t move without different friction coefficients in different directions! • Snakes manipulate friction using their scales

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