# physics

Niagara Falls are 50m high. Calculate the potential energy of 5kg of water at the top,relative to the bottom. What is the kinetic energy of this water just before it reaches the bottom, and what happens to the bottom, and what to the energy after the reaches the bottom?

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1. energy at the top: mgh=5*9.8*50 joules
energy at the bottom (ignoring energy loss on the way)=1/2 m v^2
or KE=5*9.8*50
this energy turns to heat, and warms the water.
Lord Kelvin wanted to demonstrate this, he created a large thermometer, to measure the temperature of the water at the top, then run to the bottom and measure it there (Lucerne, Switzerland). To his amazement, the water at the bottom was COOLER, not warmer. This lead to his discovery that evaporation of the water on the way down took energy (evaporation is endothermic), so water at the bottom was always cooler.

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