Machine

C

Efficiency

Another factor that people sometimes compute for machines is their efficiency, or the ratio of the work that results to the amount of work put into the machine. The efficiency of a machine is usually expressed as a percentage and can vary from 5 percent to 95 percent. A perfect machine would be 100 percent efficient. Most simple machines are very efficient, but they always lose some efficiency due to friction. An automobile engine is much less efficient because much of the energy used to move the crankshaft is lost to friction in the form of heat dissipating from the engine.

III

Types of Simple Machines

The four simple machines each function in different ways, but they all change the direction or the amount of effort put into them. All four of these machines can be used to decrease the amount of force needed to do work or to change the direction of the force. The wheel and axle and some levers can also be used to increase the speed of performance of a task, but doing so always increases the amount of force needed.

A

Inclined Plane

Ramps and staircases are simple examples of inclined planes. An inclined plane is an object that decreases the effort needed to lift an object by increasing the distance over which the effort is applied. This increase in distance allows a person to move a large object to a certain height while applying less force than would otherwise be needed. (Without the plane, a person would need to lift with a force equal to the entire weight of the object.) The tradeoff is that with the inclined plane, the person must move the object a farther distance. An inclined plane also changes the direction—from straight up to along the angle of the plane—of the effort applied. The amount of work done is the same whether the person lifts the object straight up or along an inclined plane.

The MA of an inclined plane equals the length of the plane divided by the height to which the object is raised. A long inclined plane at a small angle has a greater mechanical advantage than a steep inclined plane, because the effort is applied over a greater distance. A wedge is a double inclined plane, with a plane on each side. Wedges are often used to split wood, changing the downward direction of the force from a sledgehammer to a sideways force toward the wood being split.

---

---

A screw is a form of inclined plane in which the plane is wrapped around an axis, or pole. The MA of a screw is related to the pitch of the threads (the distance along the axis of the screw from one thread to the next) and the diameter of the axis. There are two different types of screws: fastening screws and lifting screws. Fastening screws are used to join things together. Examples of fastening screws are wood or metal screws, which have threads that dig into the materials being joined. The materials are held together by a combination of friction on the threads and compression of the screw by the materials. Other screws, sometimes called machine screws or bolts, have threads that are matched by the threads on the inside of a nut.

Lifting screws are used to lift loads or to exert forces on other bodies. An example of a lifting screw is the screw jack used to change tires on a car. Lifting screws are usually lubricated to reduce friction, but some friction with lifting screws is helpful so that the screw can safely hold the load.

B

Lever

One of the most commonly used simple machines is the lever. A seesaw is an example of a lever. The human arm is actually a lever, and the muscles apply the force needed to lift weight or move objects. A lever consists of a bar that rotates around a pivot point, which is called the fulcrum. The force applied by the user is the effort. The object being lifted is called the load. There are three classes of levers, which vary in the placement of the effort, the load, and the fulcrum along the bar. In a Class 1 lever, the fulcrum lies between the effort and the load, as in a seesaw. In a Class 2 lever, the fulcrum lies at one end, the effort is applied at the other end, and the load is in the middle, as in a wheelbarrow. In a Class 3 lever, the fulcrum is again at one end, but the load is at the other end, and the effort is applied in the middle. The human forearm is a Class 3 lever. The elbow is the fulcrum, and the forearm muscles apply the effort between the elbow and hand. Tweezers are another example of a Class 3 lever.

One of the limitations of levers is that they only operate through relatively small angles. The MA of a lever is the distance from the fulcrum to the point where the force is applied divided by the distance from the fulcrum to the load. The MA is maximized when the load is close to the fulcrum and the effort is far from the fulcrum. In this case, a small effort can move a large load.

C

Pulley

The pulley is a special type of wheel, called a sheave, which has a groove cut into the edge to guide a rope, cable, or chain. Pulleys are used at the top of flagpoles and in some types of window blinds. If a single pulley is used, the mechanical advantage is 1, and the only advantage of using the pulley is that the direction of the force needed is changed. For example, to raise window blinds, a downward pull on a cord is required.

When multiple pulleys are combined (in what is called a block and tackle), they can have mechanical advantages greater than 1, because they increase the distance the rope travels, thereby increasing the distance over which the effort is applied. The MA of a block and tackle is equal to the number of strands of rope on the part of the block and tackle that is attached to the load. Using a combination of pulleys that results in three strands of rope attached to the load requires the user to pull the rope three times farther than the load actually moves. This results in an MA of 3, which means that one-third as much effort is required to move the load. The rope on a pulley causes a good deal of friction, and this limits the number of pulleys that can be used.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain