A compound machine is a machine composed of two or more simple machines. Common examples are bicycles, can openers and wheelbarrows. Simple machines change the magnitude or direction of a force without any motor.
Simple machines are generally easy to understand and work on simple principles. They include such things as levers, which grant mechanical advantage, and wedges, which redirect relative motion, and wheels of various sorts, from pulleys to bicycle gears. These, put together, form a compound machine.
One of the simpler and older compound machines is a wheelbarrow. The classic wheelbarrow consists of two levers – its handles, which give mechanical advantage in lifting their load. They also have a wheel, which translates forward motion into rotary motion, decreasing frictional resistance compared to dragging. Another simple example is a pair of scissors. This consists of two levers with a class one fulcrum, which is a fulcrum that changes the direction of motion.
Compound machines have multiple parts that interact with each other, and at least some of them are in relative motion during use. This invariably causes internal friction, so compound machines lose efficiency when compared to simple machines. This is why many compound machines require lubrication. On the other hand, a compound machine grants the total mechanical advantage of all of its constituent parts, so it often has much greater capabilities.