Work is the transfer of Energy.

Remember the Law of Conservation of Energy? It can't be created or destroyed but when it is transformed from one form to another we say that "WORK has been done". Work is done whenever energy is passed from one thing to another.

It is labeled in Newton-meters and sometimes in Joules. Joules are a unit of energy (like Calories but much smaller... there are 4180 joules in every Calorie)

 

 

WORK has a three part definition:

All three of the following must take place for work to be done.

 
1. A force must be applied to an object
2. The object must move

3. The object must move in the same direction as the force

Look at the examples below. Is work being done or not?

Pushing a box?
Walking with a backpack
Lifting a bag of groceries
Carrying a bag of groceries

Mathematically we say...

Work = Force X Distance

 

Three formulas rolled into one!

If you know two of the three variables you can ALWAYS find the third. This is called algebra but you can use the triangle to help you along until you know all the rules.

Cover up the variable you need to find. What is left? If one is on top of the other you will divide. If they are side by side you will multiply.

W = F X D

F = W/D

D = W/F

 

 

Know the relationship between Energy and Work
1 Newton meter of work = 1 Joule of Energy

For every 1 Joule of energy you eat you are able to do 1 Newton-meter of work.

No energy...no work!

Here is an example activity from class

Determine what it takes to work off a large McMeal.

Each Calorie is the same as 4,180 Joules of energy. That means you will need to do 4,180 N-m of work to "burn" that off.

We spend an average of 2,000 Calories a day just staying alive. Most of those Calories are spent making heat (we are warm-blooded critters that need to maintain a constant body temperature. Any energy you eat or drink beyond your 2,000 or so Calories for the day will need to be "worked off" with exercise.

A pound of fat is storing about 3,500,000 joules of energy. That means you must DO 3,500,000N-m of work to "work" it off!

 

POWER is the rate at which work is done. In other words, how FAST is work getting done?

Power is measured in Watts.

There are three ways to increase your power

Decrease the time it takes to do the work
Increase the force used to do the work
Increase the distance used do the work

 

 

Machines make work easier but the same amount of work is being done.

work = force X distance regardless of how you get there.

If a 100N box needed to go up 1 m the work would be 100 N-m. What if you used a machine that cut your effort in HALF? Now you are lifting with 50 N. What # multiplied by 50 gives you 100? The answer is 2.

force
distance
Notice that as the FORCE decreases (by using a machine) the DISTANCE you travel is increased.
force
distance
100 N
1 m
10 N
10 m
50 N
2 m
5 N
20 m
25 N
4 m
4 N
25 m
20 N
5 m
2 N
50 m
10 N
10 m
1 N
100 m

 

The trade-off for making work easier (needing less force) is that you must travel a larger distance.

A machine may seem like it gives you something for nothing but that isn't possible in the world of physics. There is ALWAYS a tradeoff.

Imagine going to a special bank that takes your deposit of $1 and put $100 into your account. That would be COOL! You invested only one and got one hundred out of the deal. You would be silly not to go to that "bank" and help your pocketbook feel 100 times stronger.

Unfortunately that bank is always 100 times the distance away compared to your normal bank. In order to get the benefits of this special deal you MUST invest a greater distance to get there.

 

Machines can never be 100% efficient.

Each of the six simple machines has moving parts ...that means there is ALWAYS FRICTION to overcome!!

Friction lowers the EFFICIENCY of the machine.

You do work on the machine (work input) and then the machine does work on the object (work output). In a perfect machine these would be the same, but there are NO perfect machines. In reality, you will always have a greater INPUT than the machine will have OUTPUT.

If a machine is sticking or is rusty there will be more friction to overcome before the machine can "do its thing".

We use lubricants like silicone or oil to cut down on friction and get as much WORK OUTPUT as possible.

WORK OUTPUT the small number
X 100 = % Efficiency
________________________
WORK INPUTthe large number

 

 

 

 

Machines can help you do work in TWO possible ways
Sometimes a machine will change the direction of the user's force. This makes the work easier to do.

Machines give the user a "mechanical advantage". It multiplies the person's effort and makes you feel stronger.

(It is possible for a machine to NOT be a help and make your effort force more. We would call this a mechanical disadvantage)

 

There are TWO main categories of machines

 

Inclined planes

 

 

Levers

 

There are 6 types of simple machines

ramp

wedge
screw
lever
wheel & axle
Pulley

 

Inclined planes

ramps

The purpose of a ramp is to move up and down a distance easier than climbing or falling.

Which of these ramps would be easiest to go the same height?

What is the tradeoff for making work easier?

The mechanical advantage goes up when the ramp gets LONGER and smaller in angle.

We use them when we load trucks...

or use a wheelchair...

or sled down a hill.

 

wedges

The purpose of a wedge is to split or separate. When we sharpen a wedge we make the angle smaller and increase the mechanical advantage.

blade of the axe

 

doorstop

 

screw

The purpose of a screw is to raise or lower objects often to hold them together.

Mechanical advantage of a screw can be calculated by dividing the height of the the threads INTO the length of the threads.

 

 

Levers - can increase the size or direction of the effort force

 

How do the parts of a lever increase MECHANICAL ADVANTAGE?
Need the MA of a lever? There are three pairs to compare. Pick one and use the triangle.

FORCES - compare the amount of force you put into the machine with what it gives out

 

The BEST levers have a SMALL EFFORT FORCE and a LARGE RESISTANCE FORCE !!!

 

ARMS - compare the size of each side of the fulcrum

 

The BEST levers have a LONG EFFORT ARM and a SHORT RESISTANCE ARM !!!

 

DISTANCES - compare the height that each side of the fulcrum goes up and down

 

The BEST levers have a LONG EFFORT DISTANCE and a SHORT RESISTANCE DISTANCE !!!

 

1st, 2nd, and 3rd Class levers

The license plate above is meant to reminder you of what you'll find in the middle of each type of lever.

1st Class Levers

A FIRST CLASS LEVER has the FULCRUM in the middle.

It ALWAYS changes the direction of the force.

It will SOMETIMES be a Mechanical Advantage.

Long Effort Arm = helpful

Short Effort Arm = NOT helpful

examples of 1st class levers include:

see saw

pry-bar or the claw of a hammer

Using an oar to row a boat

2nd Class Levers

A SECOND CLASS LEVER has the RESISTANCE in the middle.

It DOES NOT change the direction of the force.

It is ALWAYS an advantage because the effort arm is always longer than the resistance arm.

examples of 2nd class levers include:

wheelbarrow

stapler

nutcracker / crableg clamp

 

 

3rd Class Levers

A THIRD CLASS LEVER has the EFFORT in the middle. It NEVER changes the direction of the force.

It DOES NOT change the direction of the force.

It is NEVER AN ADVANTAGE because the effort arm can never be longer than the resistance arm.

most of the human body
sporting equipment

Most of the bones in your body move because of third class levers. Though not a mechanical help, third class levers allow us a GREATER REACH and allow us to move things FAST.

A gorilla exactly your size would still be stronger than you because their forearms are attached with a GREATER MECHANICAL ADVANTAGE. The great apes have a longer EFFORT ARM than humans do. Think about the posture of an ape. The reason it cannot stand as straight as you is because it has more leverage in the arms and legs.

 

 

Wheel & Axle - make things turn easier OR faster

A wheel works just like lever except that goes in 360 degrees.

MA of a wheel = radius of the wheel / radius of the axle

When would you need a wheel with a larger mechanical advantage?

compare the steering wheel for a sports car ...

...with the steering wheel for a school bus...

...with the steering wheel for a pirate ship.

A larger wheel to axle ratio allow you to use less force though it is a longer distance to turn it all the way around.

 

Pulley- can increase the size and/or direction of the effort force
Calculating Mechanical Advantage

1. count the number of pulleys

(except on the zipline)

2. count the number of "supporting strings"

 

Anatomy of a Pulley

 

A single pulley can be fixed or movable.

 

How to build a block and tackle

1. Attach the top pulley (the block) to the ceiling

2. Hang a weight to the bottom pulley (the tackle)

DO NOT EVER LIFT UP, TAKE OFF, OR LET THE WEIGHTS HIT THE FLOOR.

THE TENSION WILL GO AWAY.

The pulley will likely twist, flip, fall, or do all three!

.

3.Attach a loop of the string to the hook at the bottom of the block.

4.Thread the string under the bottom pulley and pull all the slack through.

5.Thread the string OVER the top pulley and pull all the slack through.

6.Repeat as necessary for larger block and tackle pulleys.
..