Using MachinesChapter 14.2Pgs 432-436

SPI 0707.11.2

Learning Objectives

• Explain how a machine makes work easier.• Calculate mechanical advantages and

efficiency of a machine.• Explain how friction reduces efficiency.

New Vocabulary

• Input force• Output force• Mechanical advantage• Efficiency

What is a machine?

• Machines make work easier for us • Machines do not decrease the

amount of work we need to do• We still exert a force over some

distance to use machines– Raking leaves: you exert a force on a

rake to gather leaves– Using a wheelbarrow: you exert a

force to lift the handles of the wheelbarrow

Input & Output Forces

• The force that you apply to a machine is the input force.

• The work you do on the machine is equal to the input force times the distance over which your force moves the machine

• The work you do on the machine is input work• The machine also does work by using force to move an

object• The force the machine applies is the output force.• The work the machine does is output work.

Using a Machine

• When you use a machine, the output work can never be greater than the input work

• The machine makes the work easier by:– Changing the amount of force you need to exert– Changing the distance over which force is

exerted, or– Changing the direction in which you exert your

force

Changing Force

• Some machines make work easier by reducing the force you apply to complete a task

• These machines increase input force so output force is greater

• The number of times a machine increases the input force is the mechanical advantage of the machine

Calculating Mechanical Advantage

• Mechanical advantage is the ratio of output force to input force

• Can be calculated using this equation:• Force is in newtons

• Applying math pg 433 (1 &2)• Mechanical advantage does not have any units

because it is a ratio with two numbers of the same units

Changing Distance

• Changing Distance (figure 5 pg 434)– Some machines allow you to exert

a force over a shorter distance– The output force is less than the

input force– Example: using a rake– The mechanical advantage of this

type of machine is less than one because output is less than the input

Changing Direction

• Changing Direction (pg 434, fig.5)– Sometimes its easier to apply a

force in a certain direction– Example: raising a flag– Neither force nor distance is

changed – The mechanical advantage of

this type of machine is 1 because both forces are equal

Efficiency

• Machines can make output force greater than input force but they cannot make OUTPUT WORK greater than INPUT WORK

• When you use a machine there is friction between moving parts

• Friction converts some input work to thermal energy & the rest is converted to output work

• Output work is always less than the input work• Efficiency is the ratio of output work to input work

Friction

• Imagine pushing a heavy box up a ramp• The surfaces of the two objects are in contact• At points along each surface, atoms and

molecules stick and bond together• These bonds remain until a force breaks them• To keep the box moving, force must be applied

to break the bonds as they continue to form

Assess What You’ve Learned

• Identify three specific situations in which machines make work easier.

• Why does the output force exerted on a rake have to be less than the input force?

• How does the efficiency of an ideal machine compare with the efficiency of a real machine?

• How does friction reduce the efficiency of machines?

• Applying math pg 436, 6-7