SECTION 14–4: SIMPLE MACHINES

OBJECTIVES

Name, describe, and give an example of each of the six types of simple machines.

Describe how to determine the ideal mechanical advantage of each type of simple machine.

Define and identify compound machines.

SIMPLE MACHINES

There are six types of simple machines: the lever, the wheel and axle, the inclined plane, the wedge, the screw, and the pulley.

Many times you can find these simple machines in more complex machines.

LEVERS

A lever is a rigid bar that is free to move around a fixed point.

The fixed point the bar rotates around is the fulcrum.

There are three categories of levers, and they are based on the locations of the input force, the output force, and the fulcrum.

The input arm of a lever is the distance between the input force and the fulcrum. The output arm is the distance between the output force and the fulcrum.

The ideal mechanical advantage of any lever is found by dividing the input arm by the output arm.

FIRST-CLASS LEVERS

The position of the fulcrum identifies a first-class lever. The fulcrum of a first-class lever is always located between the input force and the output force.

Depending on the location of the fulcrum, the mechanical advantage of a first-class lever can be greater than 1, equal to 1, or less than one.

EXAMPLES OF FIRST-CLASS LEVERS

SECOND-CLASS LEVERS

In a second-class lever, the output force is located between the input force and the fulcrum. An example is a wheelbarrow.

The input distance your hands move to lift the wheelbarrow is larger than the output distance the wheelbarrow moves to lift its load.

The mechanical advantage of a second-class lever is always greater than 1.

THIRD-CLASS LEVERS

The input force of a third-class lever is located between the fulcrum and the output force.

The output distance over which the third-class lever exerts its force is always larger than the input distance you move the lever through. The mechanical advantage of a third-class lever is always less than 1.

Examples include golf clubs, baseball bats, and hockey sticks.

WHEEL AND AXLE

A wheel and axle is a simple machine that consists of two disks or cylinders, each one with a different radius. A steering wheel is an example of a wheel and axle.

To calculate the ideal mechanical advantage of the wheel and axle, divide the radius (or diameter) where the input force is exerted by the radius (or diameter) where the output force is exerted.

A wheel an axle can have a mechanical advantage greater than 1 or less than one.

INCLINED PLANES

An inclined plane is a slanted surface along which a force moves an object to a different elevation. A ramp is an example of an inclined plane.

The distance along the ramp is the input distance, whereas the change in height of the ramp is the output distance.

The ideal mechanical advantage of an inclined plane is the distance along the inclined plane divided by its change in height. For instance, a 6-meter-long ramp that gains 1 meter of height has an ideal mechanical advantage of 6.

WEDGES

A wedge is a V-shaped object whose sides are two inclined planes sloped toward each other.

Wedges have a mechanical advantage of greater than 1.

A thin wedge of a given length has a greater ideal mechanical advantage than a thick wedge of the same length.

Examples of wedges include knife blades and zippers.

SCREWS

A screw is an inclined plane wrapped around a cylinder.

Screws with threads that are closer together have a greater ideal mechanical advantage.

The thread on a screw is usually measured in threads per inch or threads per centimeter. A screw with fewer threads per inch takes fewer turns to drive into a piece of wood or other material, but you have to put more force to drive it into the material.

PULLEYS

A pulley is a simple machine that consists of a rope that fits into a groove in a wheel. Pulleys produce an output force that is different in size, direction, or both, from that of the input force.

The ideal mechanical advantage of a pulley or pulley system is equal to the number of rope sections supporting the load being lifted.

There are three types of pulleys: fixed, movable, and a pulley system.

FIXED PULLEYS

A fixed pulley changes only the direction of the input force.

The magnitude of the force is unchanged, and the rope lifts the load up as far as you pull down the rope. Thus, the ideal mechanical advantage of a fixed pulley is always 1.

MOVABLE PULLEYS

Movable pulleys change both the direction and the size of the input force.

A movable pulley is attached to the object being moved rather than to a fixed location. They are used to reduce the input force needed to lift a heavy object.

PULLEY SYSTEMS

Pulley systems are made up of both fixed and movable pulleys. By combining fixed and movable pulleys in a system, a large mechanical advantage can be achieved.

COMPOUND MACHINES

A compound machine is a combination of two or more simple machines that operate together.

Many familiar compound machines, such as a car, a washing machine, or a clock, are combinations of hundreds or thousands of simple machines.

VOCABULARY

Lever

Fulcrum

Input arm

Output arm

Wheel and axle

Inclined plane

Wedge

Screw

Pulley

Compound machine