Chapter 5

Lesson 2

What is a machine?A machine is a device that

makes doing work easierMachines can be simple. Some, like knives, scissors,

and doorknobs, are used everyday to make doing work easier.

Making Work Easier Machines can make work easier by

increasing the force that can be applied to an object.

A second way that machines can make work easier is by increasing the distance over which a force can be applied.

Machines can also make work easier by changing the direction of an applied force.

Increasing ForceA car jack is an example of a

machine that increases an applied force.

The upward force exerted by the jack is greater than the downward force you exert on the handle.

Increasing ForceHowever, the distance you

push the handle downward is greater than the distance the car is pushed upward.

The jack increases the applied force, but doesn't increase the work done.

Force and Distance The work done in lifting an object

depends on the change in height of the object.

The same amount of work is done whether the mover pushed the furniture up the long ramp or lifts it straight up.

If work stays the same and the distance is increased, then less force will be needed to do the work.

Changing DirectionSome machines change the

direction of the force you apply.

The wedge-shaped blade of an ax is one example.

The Work Done by MachinesWhen you use an ax to split

wood, you exert a downward force as you swing the ax toward the wood.

The blade changes the downward force into a horizontal force that splits the wood apart.

The Work Done by MachinesWhen you use a machine

such as a crowbar, you are trying to move something that resists being moved.

If you use a crowbar to pry the lid off a crate, you are working against the friction between the nails in the lid and the crate.

The Work Done by MachinesYou also could use a crowbar

to move a large rock In this case, you would be

working against gravity—the weight of the rock.

Input and Output ForcesTwo forces are involved when a

machine is used to do work.The force that is applied to the

machine is called the input force.

Fin stands for the effort force. The force applied by the

machine is called the output force, symbolized by Fout.

Input and Output Forces Two kinds of work need to be

considered when you use a machine—the work done by you on the machine and the work done by the machine.

The work done by you on a machine is called the input work and is symbolized by Win.

The work done by the machine is called the output work and is abbreviated Wout.

Conserving Energy When you do work on the machine,

you transfer energy to the machine. When the machine does work on an

object, energy is transferred from the machine to the object.

The amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine.

WorkConservation of Energy

can never get more work out than you put in

trade-off between force and distance

Win = Wout

Fe × de = Fr × dr

Ideal MachinesSuppose a perfect machine

could be built in which there was no friction.

None of the input work or output work would be converted to heat.

For such an ideal machine, the input work equals the output work.

Ideal MachinesSuppose the ideal machine

increases the force applied to it.

This means that the output force, Fout, is greater than the input force, Fin.

Recall that work is equal to force times distance.

Ideal Machines If Fout is greater than Fin, then Win and Wout can be equal only if the input force is applied over a greater distance than the output force is exerted over.

Mechanical AdvantageThe ratio of the output force

to the input force is the mechanical advantage of a machine.

The mechanical advantage of a machine can be calculated from the following equation.

Force

Effort Force (Fe)force applied to the machine“what you do”

Resistance Force (Fr)force applied by the machine“what the machine does”

Mechanical AdvantageMechanical Advantage (MA)

number of times a machine increases the effort force

e

r

FF

MA

MA > 1 : force is increasedMA < 1 : distance is increasedMA = 1 : only direction is changed

Mechanical AdvantageWindow blinds are a machine

that changes the direction of an input force.

A downward pull on the cord is changed to an upward force on the blinds.

Mechanical AdvantageThe input and output forces

are equal, so the MA is 1.

Ideal Mechanical AdvantageThe mechanical advantage

of a machine without friction is called the ideal mechanical advantage, or IMA.

The IMA can be calculated by dividing the input distance by the output distance.

EfficiencyEfficiency

measure of how completely work input is converted to work output

100%WW

Efficiencyin

out

always less than 100% due to friction

Calculating Efficiency In an ideal machine there is

no friction and the output work equals the input work. So the efficiency of an ideal machine is 100 percent.

The efficiency of a real machine is always less than 100 percent.

Increasing EfficiencyMachines can be made more

efficient by reducing friction. This usually is done by adding a lubricant, such as oil or grease, to surfaces that rub together.

A lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily.

Mechanical Advantage A worker applies an effort force of

20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA?

GIVEN:

Fe = 20 N

Fr = 500 N

MA = ?

WORK:MA = Fr ÷ Fe

MA = (500 N) ÷ (20 N)MA = 25

MA

Fr

Fe

Mechanical Advantage Find the effort force needed to lift a

2000 N rock using a jack with a mechanical advantage of 10.

GIVEN:

Fe = ?

Fr = 2000 N

MA = 10

WORK:Fe = Fr ÷ MA

Fe = (2000 N) ÷ (10)

Fe = 200 N

MA

Fr

Fe