Waves transfer energy outward equally. Since there is no barrier to the water, when a raindrop hits the water that energy of the raindrop falling has to go somewhere, the energy is transferred from the raindrop to the water and goes outward from the epicenter (where the raindrop fell). The waves (circles) will continue to travel an equal distance unless there is an outside force such as wind, another object, ect. to stop the wave
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Speed upstream(S.u) = 20/5 => 4miles/hr Speed downstream(S.d) = 10/2 => 5miles/hr Speed of man in still water(speed of boat in still water)= 1/2 * (S.u + S.d) = 0.5 * (4 + 5) = 0.5 *9 = 4.5miles/hr The speed of man in still water is 4.5 miles/hr
Since the distance downstream (with the current) equals the distance upstream (against the current), and if we: Let B stand for the speed (rate in mph) of the boat in still water, and using the formula rate X time = distance, the equation will be: (B+7) x 3 = (B-7) x 5 3B + 21 = 5B - 35 56 = 2B B = 28 mph Traveling downstream, the current will cause the boat to go faster so the 7 mph current is added to the boat's still water speed. Traveling upsteam the current slows or decreases the boat's rate so the current's speed is subtracted from the boat's still water speed.
This is a pretty straightforward calculation. By definition, a BTU is the amount of energy required to raise one pound of water one degree F. But you have one gallon of water, which weighs approximately* 8.34 pounds. So, you'd need 8.34 BTU to increase one gallon of water one degree F. Note how the amount of time was not important. Whether you heat the water slowly or quickly doesn't matter. You will still require 8.34 BTU to raise the temperature of a gallon of water one degree F. * I say approximately because the weight of water varies slightly with its temperature. Water is at its densest at 4 degrees Celsius (39 degrees F). A gallon of water at temperatures above and below that value will weigh less.
35 mph
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