February 22, 2011February 22, 2011Define: (do not create a vocab. square)MachineInput forceOutput force

MachinesMachinesWhat is a machine?A device that allows you to do

work in a way that is easier for more effective.

What are the machines that Mrs. Bradley is holding up? Can you come up with any more machines that we use everyday?

How Machines Do Work?How Machines Do Work?Machines work by changing at

least one of three things:◦Change the amount of force you

have to exert to do the work◦Change the distance over which you

exert the force◦Change the direction in which you

exert the force

How Do Machines Work?How Do Machines Work?Why does changing one of those

3 things make work easier?To understand that, we have to

look at input and output force.Input force is the amount of force

you have to put into a machine to get the work that you want.

Output force is the amount of force you get out of the machine.

How Machines Do Work?How Machines Do Work?Remember that force has two

components, strength and direction.When you put force into a machine

you are putting in the force of a certain strength in a certain direction.

The output force of the machine will not be the same as the input force in either strength, direction, or both.

Why is changing the amount or direction of a force helpful?

How Do Machine’s Work?How Do Machine’s Work?Look at Mrs. Bradley’s demonstration

and see if you can figure out why you might want to change the force strength or direction to do work.

When the machine that Mrs. Bradley created with the pencil and string changed the direction of her force, it actually increased the amount of force needed, so why would it still be helpful?

How Do Machines Work?How Do Machines Work?Sometimes, as in Mrs. Bradley’s

machine, the most important thing is to change the direction.

Imagine trying to move a very large object. Would it be easier to pull up on the object or pull down on a string that then pulled up on the object for you?

Most of the time it is easier to pull down than up.

How Do Machines Work?How Do Machines Work?Let’s review:A machine makes work easier or

more effective.Machines work by either changing

the amount of force, the direction or the force, or the distance of the force.

For example, when you use a shovel, it takes a larger force over a short distance and changes it to a smaller force over a large distance which allows you to move the dirt.

See figure 7 on page 413.

Exit Slip – (2 people per Exit Slip – (2 people per group)group)Using the magazines provided – find

and cut out several pictures (3 to 5) of machines.

Glue them onto a piece of copier paper.

Label the input and the output force for each machine.

Turn in before leaving. Make sure both of your names are

on it.

February 23, 2011February 23, 2011Define (no vocab. Square):Input workOutput workMechanical Advantage

How Machines do workHow Machines do workRemember, work is the force times the distance, so:

Input work is the input force times the input distance and

Output work is the output force times the output distance.

How Machines Do WorkHow Machines Do WorkRemember, machines will either

increase the input force, change the direction of the input force or change the distance of the input force.

Now lets look at the overhead to see the ways machines make work easier.

Mechanical AdvantageMechanical AdvantageA machine’s mechanical advantage

is how many times the machine increases the force exerted on it.◦When we exert a force on a machine,

it is very helpful when that machine is able to increase our force – it makes the work easier.

To calculate Mechanical Advantage – you have to divide the output force by the input force.

Calculate Mechanical Calculate Mechanical AdvantageAdvantageMechanical Advantage = Output

forceInput force

A machine that increases force will have a larger mechanical advantage than a machine that increases distance but not force.

A machine that only changes direction should always have a mech. adv. = 1

Calculate Mechanical Calculate Mechanical AdvantageAdvantageIncreasing force –

◦Output force = 30N◦Input force = 10N◦Mechanical Advantage = 30N/10N = 3

Increasing Distance –◦Output force = 10N◦Input force = 20N◦Mechanical Advantage = 10N/20N = 0.5

Note: a mech. adv. less than one is still useful

Efficiency of MachinesEfficiency of MachinesDo we get out of machines everything

that we put into them or is something lost along the way?

Efficiency compares the output work to the input work.

Normally, in the real world, the output work is less than the input work because some work is lost to friction.

Efficiency is always expressed as a percent

Real vs. Ideal MachinesReal vs. Ideal MachinesMachines with an efficiency of

100% would be Ideal machines. ◦No machines exist in the real world due

to friction.◦A machine’s Ideal mechanical advantage would be its mechanical advantage with an efficiency of 100%.

◦A machine’s actual mechanical advantage is its measured mechanical advantage.

Exit SlipExit SlipIn your notebooks, do the section

assessment questions on page 419. Do not do the math practice square b/c we should have already done those in class.

Show me your questions before you leave today.

We will go over them if we have time.