A dock is 6 feet above water. Suppose you stand on the edge of the dock and pull a rope attached to a boat at the constant rate of 2 ft/s. Assume that the boat remains at water level. At what speed is the boat approaching the dock when it is 20 feet from the dock? 10 feet from the dock? Isn’t it surprising that the boat’s speed is not constant?
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Expert's answer
2021-10-12T02:58:47-0400
Apply the Pythagorean theorem:
x2+h2=L2 where h=6ft
Now apply implicit differentiation:
2xdtdx=2LdtdL⇒dtdx=xLdtdL
For x=20ft,L=(202+62)
⇒dtdx=20436×(2)=2.08ft/sec
For x=10ft,L=(102+62)
⇒dtdx=10136×(2)=2.33ft/sec
Result: Speed of boat increases as it gets closer to dock.
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