Chapter 4 Work and Machines

DO NOW  Which of the following requires the most work and why?  1. The work required to run a half hour on a treadmill  2. The work to walk 5 miles with a 30 lb backpack on your back (work on the backpack)  3. The work to lift a bag of groceries from the ground to the counter. (work on the bag of groceries)

The answer was:  3. The work to lift a bag of groceries from the ground to the counter. (work on the bag of groceries)

So… what is the meaning of “work” anyway?  Work is when you exert a force on an object that causes the object to  1. move some distance  2. in the same direction as the force.

Calculating WORK  Work = force x distance  What is force measured in?  Newtons  What is distance measured in?  meters  Work = Newton-meters (N*m)  1 Newton-meter = 1 Joule

DO NOW  Audrey and Henry get home from school and go upstairs to get ready for the soccer game. Audrey runs up the steps as fast as she can. Henry slowly meanders up the stairs.  Who did more work?

Well…  They did the same amount of work. Time has nothing to do with the amount of work done. Maybe that seems unfair …… BUT …. Audrey does have one up on Henry. Audrey ran up the stairs with more power.

 Power is the amount of work done on an object in a unit of time.

Scenario 2  Audrey and Henry live in a pretty huge house with 2 flights of stairs. Audrey runs up 2 flights in the same amount of time it takes Henry to run up one flight.  Who does more work? Audrey does more work. She also has more POWER!

 To have more power:  1. do the same work in LESS time  2. do MORE work in the same time

Power = work time Power = (Force x distance) time Power = (Newtons x meters) seconds Power = Joules seconds Power = Watts! 1 Watt = 1 Joule/second

Talking about Power  More often, people use “kilowatts” and “horsepower” when talking about power.

 Can opener  Ball  Corkscrew  Pencil  ruler  Pliers  Book  Screwdriver  Chalk  Paper Look at and copy the following list and label as “machine” or “not machine”. Why did you classify some things as machines? Machines  can opener, corkscrew, pliers, screwdriver

What is a Machine? A device that makes work EASIER but DOESN’T MEAN THAT YOU DO LESS WORK!

A Machine Makes work easier by: 1. changing amount of force you need to exert 2. changing the distance over which you exert your force 3. changing the direction in which you exert your force

When using a machine, you must put a force into it  The force you exert on a machine is the input force. Input force You will have to apply your input force through an “input distance”

The machine then in turn exerts a force on some object.  The force the machine exerts on an object is called the output force. Output force The machine will apply the force through an “output distance”

Your input force x input distance =

The machine applies an Output force x output distance =

GoodTrade-offExample: Changing Force Only need to use small input force Need large input distance Ramp, faucet knob, low gears on bike Changing Distance Only need small input distance Need large input force Chopsticks, hockey stick, high gears on bike Changing Direction Direction of input force is made more convenient Same amount of force is required. Weight machine, pulley system

Mechanical Advantage  How many time does a machine increase your input force?  Mechanical advantage = output force input force

Mechanical advantage of machines that 1. Increase force>1 2. Increase distance<1 3. Change direction=1

DO NOW  In theory, the work you INPUT into a machine should always equal the work that the machine OUTPUTS. Why do you think that in reality, the output work always ends up being LESS?

Not-so-perfect-machines Machines should have the same output work as the input work that you put into them, but that would be an ideal situation. In reality, machines are not that efficient.

Efficiency of Machines  IT’s a percent that compares output work to input work, and it’s never 100% because of friction.

Calculating Efficiency Efficiency = output work x 100% input work

EXAMPLE: You apply 20 Joules of work into a can opener to open up a can. The can applies 15 Joules of work onto the can. Calculate the efficiency of the can opener. Efficiency = output work x 100% input work Efficiency = 15 x 100% 20 Efficiency = 75 %

IDEAL MACHINE  It would have 100% efficiency

DO NOW  Name all the simple machines you can think of!

Simple Machines

Franklin institute – simple machines   science/simple-machines/activities/simple- machines-1/ science/simple-machines/activities/simple- machines-1/  est.htm est.htm

Inclined Plane  Flat, sloped surface  Use less force over longer distance  Ideal mech advantage = length incline / height incline  Ex: ramp

Wedge  Device thick at one end and tapers to thin edge at other end  2 inclined planes bk to bk  Ideal MA = length wedge / width wedge  Ex: knife, ax, zipper, cheese grater, shovel

Screw  Inclined plane wrapped around cylinder  Only need a small force over large distance  Ideal MA – length around threads / length of screw  Ex: jar lid, screw, lightbulb

Levers  Rigid bar free to pivot on a fixed point called a fulcrum  Ideal MA = distance from fulcrum to input force distance from fulcrum to ouput force  There are 3 types of levers depending on the position of the fulcrum, input force, and output force

First Class Lever  Ex: scissors, pliers, seesaws  Pg 129

Second Class Levers  Ex: door, nutcracker, bottle opener

3 rd Class Lever  Ex: fishing pole, shovel, baseball bat, hockey stick

Wheel and Axle  Two circular or cylindrical objects fastened together that rotate about a common axis.  Wheel – object with large radius  Axle = object with smaller radius  You need a small input force but large input distance  Ideal MA = radius of wheel radius of axle

Pulley  Simple machine made of grooved wheel with a rope or cable wrapped around it  Ideal mechanical advantage = number of sections of rope that support the object  Types  Fixed  Movable  Block and tackle