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Chapter 14 Work_ Power_ and Machines

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Chapter 14 Work_ Power_ and Machines Powered By Docstoc
					Work, Power, &
  Machines
           Chapter 14
           Mr. Kovacs
Integrated Chemistry and Physics


                                   1
What is work?




• The product of the force applied to
  an object and the distance through
  which that force is applied.
                                        2
What is work?
           • According to the physics
             definition, you are NOT
             doing work if you are
             just holding the weight
             above your head.
           • You are doing work only
             while you are lifting the
             weight above your head.
           • No movement : No work
                                         3
For work to be done on an object, the
object must ___________?____________.

• move in the direction of the force.




                                        4
Work Requires Motion
• If the wall doesn't move, the prisoner
  does no work.




• No movement : No work                    5
  Work Depends on Direction
  • 1) Work must have a force
  • 2) The force must be in the direction of
    the motion

Force, F


                         distance, d

                                               6
Calculating Work
• To do work on an object you have to
  push the object a certain distance in
  the direction that you are pushing
• Work = force x distance = F x d
• If I carry a box across the room I do
  not do work on it (the box) because
  the force is not in the direction of
  the motion. Was any work done?
                                          7
Is work being done or not?
Mowing the lawn        •   YES
Weight-lifting         •   YES
Carrying groceries     •   NO
Moving furniture up    •   YES
a flight of stairs
Pushing against a      • NO
locked door
Swinging a golf club   • YES
                                 8
Is work being done or not?

•   Climbing stairs?
•   Lifting a book?
•   Pushing a shopping cart?
•   Carrying a football?
                               9
Calculating Work


All or part of the force
must act in the direction
of the movement.
                            10
Units of Work: The Joule
• 1 newton-meter
  is a quantity
  known as a
  joule (J).
• Named after
  British physicist
  James Prescott
  Joule.
                      •(1818-1889)   11
What is the SI unit of work?

Duh!!!!!
• The joule!
• Or 1 NM!

                               12
Using the Work Formula
• Work = Force x Distance
  F = 500 pounds (2000 N)
  D = 8 feet (2.5 meters)

• W = 2000 N x 2.5 m
    = 5000 N-m
    = 5000 J

                            13
Do you do more work when you
finish a job quickly?


•NO
• Work does NOT involve time, only force
  and distance.



                                           14
• How quickly work is done.
• Amount of work done per unit time.
• If two people mow two lawns of equal
  size and one does the job in half the
  time, who did more work?
• Same work. Different power exerted.
• POWER = WORK / TIME

                                          15
What does power measure?

• The rate of doing work!!!!!!
• How fast the work is done!
• Work/time

                                 16
James Watt
             • A unit named after
               Scottish inventor
               James Watt.
             • Invented the steam
               engine.
             • P = Work/time
               – Joules/second
               – 1 watt = 1 J/s


                                    17
Calculating Power: Page 415




              1.0 m




                              18
You row a boat across a pond. You do
3600 J of work on the oars in 60 seconds.
How much power did you use?

• 3600 J /60 sec = 60 J/sec = 60 W




                                            19
watts
• Used to measure
  power of light
  bulbs and small
  appliances
• An electric bill is
  measured in
  kW/hrs.
• 1 kilowatt = 1000 W
                        20
Horsepower (hp) = about 746 watts
• Traditionally associated with engines.
  (car,motorcycle,lawn-mower)




• The term horsepower was developed to
  quantify power. A strong horse could
  move a 746 N object one meter in one
  second.                                  21
What is the SI unit of power?

• Watt




                                22
How much power does a 100 watt light
bulb use if it is turned on for 30 seconds?
• One more duh!
• 100 watts!!!!!!!!!!!!!!!!!!!



                                              23
End of Section 1




                   24
Machines Do Work
• A device that makes work easier.
• A machine can change the size, the
  direction, or the distance over which a
  force acts.




                                            25
Ramps are useful machines!
           • It makes it easier to move.




             Increasing Distance
             Reduces Force

                                      26
  Increasing Force
  A ramp can reduce the force




      WORK DONE                       WORK DONE
big force  little distance   little force  big distance

                                                        27
Forces involved:

• Input Force • Output Force
   –FI          –FO
   –Force       –Force
    applied to   applied by
    a machine    a machine
                               28
Two forces, thus two types of work
• Work Input            • Work Output
 Work done on a         Work done by a
  machine                 machine
=Input force x the      =Output force x the
  distance through        distance through
  which that force acts   which the resistance
  (input distance)        moves (output
                          distance)


                                                 29
Figure 7 page 419




                    30
Can you get more work out than you
put in?




       •NO
Work output can never be greater than
 work input.
                                         31
End of Section 2




                   32
How Does Input Force
Location Affect a Machine?
A nutcracker is a machine used to make cracking
nuts easier. As shown below, use a nutcracker to
crack three nuts, each time squeezing the
nutcracker’s handles at a different location.




                                                   33
Applying force at which handle location resulted in the
nutcracker cracking the nuts the most easily?
The nutcracker worked best when force was applied at
location 1.
How does the distance from the nutcracker’s pivot point
to the point where the force is applied affect the
nutcracker’s ability to crack nuts?
The greater the distance between the pivot and the
force, the better the nutcracker was at breaking nuts.
                                                     34
Mechanical Advantage (MA)
• The number of times a machine
  multiplies the input force.




                                  35
Actual Mechanical Advantage
• ACTUAL
• Involves friction.
• Calculated the same for all machines
• Actual Mechanical Advantage = Output force/Input force




                                                           36
Ideal Mechanical Advantage
• IDEAL
• Involves no friction.
• Is calculated differently for different
  machines
• Usually input distance/output distance
  – Actual mechanical advantage is always less
    than ideal mechanical advantage.

                                                 37
Calculating Mechanical Advantages:




                                     38
Calculating Mechanical Advantages:
• MA equal to one.
     (output force = input force)
• Change the direction of the applied
  force only.




                                        39
Calculating Mechanical Advantages:
• Mechanical advantage less than one
• An increase in the distance an object is
  moved (do)




                                             40
Efficiency
• Efficiency can never be greater than
  100 %. Why?
• Some work is always needed to
  overcome friction.
• A percentage comparison of work
  output to work input.
  – work output (WO) / work input (WI)

                                         41
End of Section 3


    Thank you!




                   42
1. The Lever
• A bar that is free to pivot, or move
  about a fixed point when an input force
  is applied.
• Fulcrum = the pivot point of a lever.
• There are three classes of levers based
  on the positioning of the input force,
  output force, and fulcrum.

                                            43
First Class Levers
• Fulcrum is located
  between the effort
  and resistance.
• Makes work easier
  by multiplying the
  effort force AND
  changing direction.



                        44
First Class Levers
• Work Out = Work In
• Small force applied over large distance
  is the same as large force applied over
  a small distance.

                   F   d=F   d



                                            45
Second Class Levers
          • Resistance is found
            between the fulcrum
            and input force.
          • Makes work easier
            by multiplying the
            input force, but NOT
            changing direction.



                                   46
Third Class Levers
• Input force is
  located between the
  output force and the
  fulcrum.
• Does NOT multiply
  the input force, only
  multiplies the
  distance.
• Examples:
                          47
Mechanical advantage of levers.
• Ideal = input arm
  length/output arm
  length
• input arm =
  distance from input
  force to the fulcrum
• output arm =
  distance from output
  force to the fulcrum
                                  48
Mechanical advantage of levers.




                                  49
2. The Wheel and Axle
• A lever that rotates
  in a circle.
• A combination of
  two wheels of
  different sizes.
• Smaller wheel is
  termed the axle.
• IMA = radius of
  wheel/radius of axle.

                          50
3. The Inclined Plane
• A slanted surface
  used to raise an
  object.
• Examples: ramps,
  stairs, ladders
• IMA = length of
  ramp/height of ramp
  Can never be less
  than one.

                        51
4. The Wedge
• An inclined plane
  that moves.
• Examples: knife,
  axe, razor blade
• Mechanical
  advantage is
  increased by
  sharpening it.


                      52
5. The Screw
           • An inclined plane
             wrapped around a
             cylinder.
           • The closer the
             threads, the greater
             the mechanical
             advantage
           • Examples: bolts,
             augers, drill bits
                                    53
6. The Pulley
           • A chain, belt , or
             rope wrapped
             around a wheel.
           • Can either change
             the direction or the
             amount of effort
             force
           • Ex. Flag pole, blinds,
             stage curtain
                                      54
The Pulley




             55
Pulley types
• FIXED                • MOVABLE
• Can only change      • Can multiply an effort
  the direction of a     force, but cannot
  force.                 change direction.
• MA = 1               • MA > 1




                                              56
Page 432 Figure 19




                     57
• A combination of two or more simple
  machines.
• Cannot get more work out of a
  compound machine than is put in.




                                        58
Assignment:
• Pages 441-442
• 1-11, 13, 14, 15, 17, 19, 22,
  26, 27, 28, 29
• WB Section 4


                                  59
14.2 Work
• 5. A woman lifts her 100-newton child up one
  meter and carries her for a distance of 50
  meters to the child’s bedroom. How much
  work does the woman do?

100 N X 1 m =
100 N·m or 100 joules
Note: No work is done on the child when she
  carries it.

                                                 60
14.2 Power
5. A horse moves a sleigh 1.00 kilometer by
  applying a horizontal 2,000-newton force on
  its harness for 45 minutes. What is the power
  of the horse? (Hint: Change Km’s to m’s and
  convert time to seconds.)

45 min = 2700 s
2000 n X 1000 m / 2700 s
740.74 watts

                                                  61
14.3 Mechanical Advantage
5. A machine with a mechanical advantage of
  2.5 requires an input force of 120 newtons.
  What output force is produced by this
  machine?

2.5 = x / 120 n
X = 2.5 x 120 n

X = 300 newtons

                                                62
• For work to be done on an object, the
  object has to ____________________.




                                          63
• Any part of a force that does not act in
  the direction of an object’s motion does
  no ____________________ on an
  object.




                                             64
• The SI unit of work is the __________.




                                           65
• The rate at which work is done is called
  ____________________.




                                             66
• The SI unit of power is the _________.




                                           67
• A machine is a device that can multiply
  _____________.




                                            68

				
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