Escapement between the balance wheel and the escape wheel; this is the form used in modern watches. In 1867 Georges Frederic Roskopf invented an inexpensive, less accurate form called the Roskopf or pin-pallet escapement, which was used in cheap "dollar watches" in the early 20th century and is still used in cheap alarm clocks and kitchen timers. A rare but interesting mechanical escapement is John Harrison's grasshopper escapement invented in 1722. In this escapement, the pendulum is driven by two hinged arms (pallets). As the pendulum swings, the end of one arm catches on the escape wheel and drives it slightly
Mechanical energy isolated system remains constant in time, as long as the system is free of friction and other non-conservative forces. In any real situation, frictional forces and other non-conservative forces are present, but in many cases their effects on the system are so small that the principle of conservation of mechanical energy can be used as a fair approximation. Though energy cannot be created or destroyed in an isolated system, it can be converted to another form of energy. In a mechanical system like a swinging pendulum subjected to the conservative gravitational force where frictional forces like air drag and friction