Chapter 14 pg 412435 Todays Learning Objectives Know the vocabulary of this chapter What is the twopronged test to see if something qualifies as work Solve and calculate problems about work ID: 759510
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Slide1
Work, Power, & Machines
Chapter
14,
pg
412-435
Slide2Today’s Learning Objectives
Know the vocabulary of this chapter.
What is the two-pronged test to see if something qualifies as work?
Solve and calculate problems about work.
Solve and calculate problems about power.
Slide3Work and Power (14-1)
Work
–
When a force causes an object to move in the direction of the force
Two prong test to see if it is work:
The object must move as the force is applied
The direction of the motion is the same as the direction in which the force is applied
Slide4Work
Work or not?
Pushing a sled?
Picking up a bag of groceries?
Carrying your backpack to class?
Trying to lift a car with your pinky?
Slide5Work
Formula for work:Work = Force x distanceUnits = joule (J)1 J = 1 N•m
Slide6MathBREAK
A man applies a force of 500 N to push a truck 100 m down the street. How much work does he do?Answer: 50,000 J
Slide7Power
Power –The rate at which work is being donePower is how fast work happensUnit = watt (W)1 W = 1 J/s
Slide8MathBREAK
Which is more powerful, a motor that can do 4500 J of work in 25 seconds, or a motor that can do 2000 J of work in 15 seconds?Answer: 180 W vs. 133 W, so the first one is more powerful
Slide9Today’s Learning Objectives
What are the advantages of a machine?
Explain the force / distance trade off.
Know that work output can never be greater than work input.
Solve problems (calculate) for mechanical advantage.
Slide10Machines (14-2, 14-3)
Machine – Device that changes the size or direction of a forceEx. scissors, screw, bottle opener
Slide11Machines
Advantages of a machine
Decrease the force required from your body
or
Increase the distance (that you can control)
or
Change the direction of your force
Slide12Force / Distance tradeoff
Example 1: (changing size)You put in less force, but you must lift it farther.
Slide13Machines
Example 2: (changing direction)In a simple pulley like this, the force is the unchanged, but the direction of that force changes.
Slide14Machines
Trade off:
Force or distance can increase, but not together. When one increases, the other must decrease.
Slide15Machines
Two kinds of WorkWork input –Work you put into a machineWork output –Work done by the machine
Slide16Machines
Do machines increase the amount of work done?
No. Work output can NEVER be greater than work
input
Some of the work input is always used to overcome the friction in the machine
Machines
can increase your force though, and that makes work feel easier
Slide17Machines
Mechanical advantage –Tells you how many times the machine multiplies forceFormula:Result may be >, =, or < 1
Slide18MathBREAK
You apply 200 N to a machine and the machine applies 2600 N to an object. What is the mechanical advantage?
Answer: 13
Note, no units required on Mech. Adv.
Slide19Machines
Mechanical Efficiency –A comparison of a machine’s work output with the work inputWork output is always less than work input because of friction in the machineEffect lessened with oilFormula:
Slide20Today’s Learning Objectives
Be able to draw and label examples of the six types of simple machines.
Be able to label diagrams of the three classes of levers.
Slide21Simple Machines (14-4)
Six simple machines
Lever
Inclined plane
Wedge
Screw
Wheel and axle
Pulley
Slide22Levers
Lever – simple machine consisting of a bar that pivots at a fixed point, called a fulcrum3 kinds of levers:First classSecond classThird class
Slide23First Class Lever
First Class Lever –Fulcrum between input force and loadChange direction of forceCan increase or decrease force, depending on location of fulcrum
Slide24Second Class Lever
Second Class Lever –Load is between the fulcrum and the input forceDecrease amount of effort requiredEx. wheelbarrow
Slide25Third Class Lever
Third Class Lever –Input force is between fulcrum and loadOutput force is less than input forceDistance of force is increasedEx. human forearm
Slide26Today’s Learning Objectives
Be able to draw and label examples of the six types of simple machines.
Know an example of a compound machine and why it qualifies as compound.
Slide27Inclined Plane
Inclined Plane –Simple machine that is a straight slanted surfaceEx. ramp
Slide28Wedges
Wedge –A double inclined plane that movesEx. knife
Slide29Screws
Screw – An inclined plane that is wrapped into a spiral (Click: Big Bang quote)
Slide30Wheel and Axle
Wheel and axle –Simple machine consisting of two circular objects of different sizesEx. doorknob, volume knob
Slide31Pulleys
Pulley –Simple machine consisting of a grooved wheel that holds a rope or cableLoad on one end, input force on otherEx. used in blinds
Slide32Compound Machines
Compound machines –Machines made of 2 or more simple machinesMost machines fit this categoryEx. scissors, zippers, etc.
Slide33Work = F x D
IMA
= d
in
÷
d
out
AMA =
F
out
÷ F
in
W
in =
F
in x
d
in
W
out
=
F
out
x
d
out
Eff
. = (
W
out
÷ W
in
) x 100