تجربة جول حفظ الطاقة the conservation of energy
Suppose you want to run a marathon. A basic law of science called the conservation of energy tells us you need to fill your body with "42 km (26 miles) worth of food." Or say you want to drive a car from New York City to Los Angeles. The same law says you'll need to put about 4000km (2500 miles) of gasoline in your tank. In other words, anything you want to do needs energy to do it. The energy you need is equal to the work you want to do (and remember that "work" is the scientific name for how much effort you're putting in, which involves using a force for a certain distance).
The person who figured this out experimentally was James Prescott Joule. In his experiment, there was a large container full of water that had a paddle wheel fixed inside it. The paddle wheel was connected to an axle around which a string was wrapped many times. The string was looped over a pulley and had a heavy on the end of it. When Joule released the weight (1), it pulled the string around the pulley (2), turned the axle (3), and made the paddle wheel spin, which heated up the water. He let the weight fall about 20 times so the water heated up enough for him to be able to measure. Once he'd done all his calculations, he showed that the amount of potential energy lost by the falling weight was exactly equal to the amount of heat energy gained by the water. Energy lost = energy gained is another way of expressing the conservation of energy: you can't create or destroy energy, but you can convert it from one form into another (in this case, from potential energy into heat).
Here's a quick visual explanation of Joule's mechanical equivalent of heat experiment and the earlier scientific work of Sir Benjamin Thompson, Count Rumford, on which it was based.