Machines and Mechanical Advantage

• Machines multiply and redirect forces.
• Machines help people by redistributing the work put into them.
• They can change either the size or the direction of the input force.

• Different forces can do the same amount of work.
• A machine allows the same amount of work to be done by either decreasing the distance while increasing the force or by decreasing the force while increasing the distance.

• Mechanical advantage tells how much a machine multiplies force or increases distance.

• Mechanical Advantage Equation

Mechanical advantage = output force/ input force = input distance/ output distance

EXAMPLE

Mechanical Advantage Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.

1. List the given and unknown values.

Given: input distance = 5.0 m

output distance = 1.5 m

Unknown: mechanical advantage = ?

Write the equation for mechanical advantage.

Because the information we are given involves only distance, we only need part of the full equation:

Mechanical advantage = input distance/ output distance

Insert the known values into the equation, and solve.

Mechanical advantage = 5.0 m/ 1.5m = 3.3

The Lever Family

• The most basic machines are called simple machines.

• The six types of simple machines are divided into two families.
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• Levers have a rigid arm and a fulcrum.

• Levers are divided into three classes.
• All first-class levers have a fulcrum located between the points of application of the input and output forces.
• In a second-class lever, the fulcrum is at one end of the arm and the input force is applied to the other end.
• Third-class levers multiply distance rather than force. As a result, they have a mechanical advantage of less than 1.

• Pulleys are modified levers.
• The point in the middle of a pulley is like the fulcrum of a lever.
• A single, fixed pulley has a mechanical advantage of 1.
• Multiple pulleys are sometimes put together in a single unit called a block and tackle.

• A wheel and axle is a lever or pulley connected to a shaft.
• The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines.

• Inclined planes multiply and redirect force.
• An inclined plane turns a small input force into a large output force by spreading the work out over a large distance.

• A wedge is a modified inclined plane.

• A screw is an inclined plane wrapped around a cylinder.

• A machine made of more than one simple machine is called a compound machine.

• Examples of compound machines are:
• scissors, which use two first class levers joined at a common fulcrum
• a car jack, which uses a lever in combination with a large screw