Machines Machine: A device that helps you do work All machines are made up of one or more simple...
-
Upload
cuthbert-green -
Category
Documents
-
view
216 -
download
0
Transcript of Machines Machine: A device that helps you do work All machines are made up of one or more simple...
MachinesMachine:
A device that helps you do work
All machines are made up of one or more simple machines
Machines make work easier by changing the size or direction of force
Machines/EnergyMachines obey the law of Conservation of Energy
Energy or Energy or Friction
Work into a + work out of + losses
Machine a machine
EfficiencyEfficiency:
Comparison of work output to work input
All machines have friction losses
100% efficient machine (no friction or heat losses) does not exist
Mechanical Advantage
Mechanical Advantage (MA):
How many times easier a machine makes your work
Number of times a machine multiplies effort force
No Unit for MA
Mechanical Advantage
You want a robot with a MA of 10 instead of 5, as work will be 10 times harder rather than 5 times easier
MA = FR Resistance force
DE Applied force
MA = FE Applied Speed
DR Resistance Distance
Mechanical Advantage
If we apply 20N of force to move a 60N object
Solve problem on board
No unit on the number for MA
Forces on MachinesTwo forces involved with machines
Effort Force: (FE)
Force you put on a machine
Resistance Force: (FR)
Force the machine is working against (often the weight of the object)
Distances/MachinesEffort Distance: (DE)
Distance of the effort force
Resistance Distance: (DR)
Distance the object moves See word document (solve problems)
Lever LawLever law:
Work in = work out
F1D1 = F2D2
An 800N man is 2 meters from the fulcrum of a teeter-totter. How far away must a 400N child sit in order to balance?
Mechanical Advantage
A person pushes a crowbar down 2 m with 200N of force. The crowbar raises the box 0.3 m. What is the MA?
Show work/PE/KE/Power Eureka videos
Six simple machines
6 simple machines:
Inclined Plane
Wedge
Screw
Lever
Wheel and axle
Pulley
Inclined PlaneEvery day my lovely wife lifts 3000 pounds 5 feet. She does this 2, 4 or 6 times a day, depending on the day. What is she doing?
Driving her car up the driveway
Inclined PlaneInclined Plane:
A slanted surface used to move an object from low to high or high to low position.
Examples: driveway, ramp, stairs, road
Show Eureka Inclined plane video
Inclined Plane MA’sDemo with inclined plane, spring scale and weight
FR weight of object (lift with spring
scale)
FE spring scale drag
DE length of ramp
DR height of ramp
Slide object and do Force MA’s (MA = FR/FE)
Inclined Plane MA’sHow will MA vary with height?
Steeper inclined plane = less MA
Less steep inclined plane = large MA (easier)
Example: Climb a mountain - steep sections harder to climb
Inclined Plane LabSolve Inclined Plane problems in word document
Demo lab. Groups of 3 – 4 do lab and turn it in.
Review lab
Inclined Plane Review
Quick review of inclined plane: See word document for quality graphic and formulas
How will MA vary with height for an inclined plane?
Steeper inclined plane: less MA, more difficult to use
Less steep inclined plane: larger MA, easier to use
WedgeWedge:
An inclined plane that moves
Examples: knife, axe, door stoop, wood splitting wedge…..
How does sharpening a knife help it do more work?
ScrewScrew:
An inclined plane wrapped around a cylinder Examples: spiral staircase, mountain road,
C-clamp, bolts… Each time a screw turns, it moves a
definite distance up or down.
Depends on the distance between threads
Draw on chalkboard
LeverLever:
A bar that rotates on a fixed point called a fulcrum
Fulcrum
The fixed point a lever rotates around
MA’s can be very large, depends on leverage
Lever MATheoretically, if you have a long and rigid enough lever you could lift anything (Webber)
MA = DE = Effort arm length
DR Resistance arm length Use word document for all 3 classes of levers and examples
Lever DemosWatch Eureka video on levers
Demo lever activity Students calculate MA and list class of lever
Demo lever law with meter stick and weights
Wheel and axle demos
Wheel and AxleWheel and Axle
A lever that moves in a circle turning another circle
Examples: screw driver, socket wrench, faucet knob, steering wheel, pencil sharpener, door knob…
Wheel and Axle MACalculate MA of wheel and axle
MA = DE = Radius of wheel
DR Radius of axle
See word documents
Advantages of a Machine
What are the two advantages of a machine?
They change the size and direction of effort force
PulleysPulley
A lever that rotates around a fixed point
2 benefits of a pulley
1. Change the direction of force
2. Provide Mechanical Advantage
Bag the technology and go back to the chalk board like our ancestors
Example ProblemsExample problems:George weighs 700 N and is 5 meters from the fulcrum on a teeter-totter. How far must Martha be from the fulcrum to balance if she weighs 200 N?
F1D1 = F2D2
A 46 N force is used to lift a 192 N weight. What is the MA?
Example ProblemsA lever is used to lift a 2000 N weight with an effort force of 50 N. What is the MA of the bar?
What is the MA of a ramp 30 meters long and 4 meters high?
Example ProblemsA woman pushes a crow bar down 3 meters to pry a spike out 0.15 meters. What was the MA?
Create and solve 8 MA problems (4 with distance and 4 with force) and 2 lever law problems (teeter-totter). Turn this in for easy points.