Simple machines, such as pulleys, levers, and gears, are never 100 percent efficient. This means that the energy you put into them is always greater than the useful energy you get out of them. Why is this? Because the simple machines are human-made, they are not perfect. In the real world, at least some of the input energy becomes thermal energy because of friction. This energy is "lost" for useful purposes. Conservation of energy says the total amount of energy is constant. Because our methods of measuring output energy are not precise enough