Presentation on theme: "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."— Presentation transcript:
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”
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 %
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