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# Force & motion ( teacher background...big)

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A serious slide show on Force and Motion mainly for non-science experienced elementary teachers.

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• Ethiopian calendar of the tropics are different from Gregorian calendar of the temperates.
30 days of Meskeram is different from 30 days of September,
30 days of Tikimt is different from 31 days of October,
30 days of Hidar is different from 30 days of November,
30 days of Tahisas is different from 31 days of December,
30 days of Tir is different from 31 days of January,
30 days of Yekatit is different from 28 or 29 days of February,
30 days of Megabit is different from 31 days of March,
30 days of Miyazia is different from 30 days of April,
30 days of Ginbot is different from 31 days of May,
30 days of Sene is different from 30 days of June,
30 days of Hamle is different from 31 days of July,
30 days of Nehase is different from 31 days of August,
5 or 6 days of Pagume is different from 6 to 10 or 11 of September

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### Force & motion ( teacher background...big)

1. 1. TO SEE THE BUILT- IN ANNIMATION EFFECTS, THIS SLIDE SHOW MUST BE DOWNLOADED
2. 2. Force Force is a push or a pull and MotionBy Moira Whitehouse PhD
3. 3. The strength of a force can be measured. How strongthe push or pull is measured with a spring scale inunits called newtons.One newton is equal to about a quarterof a pound.This is a newton scale. By hooking itonto an object and pulling it along, onecan read the force that is required tomove that object under those conditions.
4. 4. There are four kinds of forces; some scientistsalso add the fifth, friction.Strong nuclearforce responsible for binding atomic Short-range force:nuclei together. The strongest of the fourfundamental forces of nature.Important force in certain decaying functions Weak nuclear force:within the atom, but way beyond me.Electromagnetic force: a force between objectsexerted by positively and negatively chargedparticles.Gravitational force: the force of attractionbetween all masses in the universe; especially theattraction of the earths mass for bodies near it.
5. 5. Electromagnetic forceWe know that everything is made upof tiny particles called atoms. Let’s start with....Although atoms are much too small tobe seen, scientists have figured outthat they are made of even smallerparticles that have electrical charges.They are called protons and electrons.
6. 6. Protons and electronsEverything is made up of atoms and every atom ismade up of protons in the nucleus with positiveelectrical charge (+) and electrons swirling aroundthe nucleus each with a negative electrical charge(-). There are also neutrons in the nucleus but theyhave no electrical charge. http://www.windows.ucar.edu/
7. 7. Because atoms have the same number ofpositive (+) and negative (-) charges, mostthings are in electrical balance. But when the atoms of an object get out of balance electrically, stra nge things happen.
8. 8. They can get out of balance when the swirlingnegatively charged electrons are knocked loose fromtheir atom. This can happen rather easily and helpsexplain static electricity.Sometimes when two electrically balanced objectsrub against each other, electrons from the one arerubbed off onto the other. The object that receivedthe electrons would then have extra electrons and anoverall negative charge.The object that lost the electrons would no longer bebalanced having too many protons for the remainingelectrons and thus becoming positively charged.
9. 9. Even though we say that “strange things” happendepending on the balance or unbalance status of theelectrons, the reactions are actually very predictable. If the both objects have excess negative charges: If the both objects have excess positive charges:If the objects are balanced: and the other has If one object has positive negative excesses: Opposite charges Uncharged attract Like charges repel
10. 10. Shoes and carpet, like most everything else, aremade of atoms that are electrically balanced. Butwhen shoes rub against the carpet, electrons aretransferred from the carpet to the shoes and theshoes become negatively charged.The carpet which loses electrons to the shoesbecomes positively charged.
11. 11. As you proceed about your business with all thoseextra electrons, you do not notice anything until......you touch a metal doorknob? Those electrons that moved up from your shoes are now ready to get “in balance” again. The metal doorknob ZAP!!! which is a good conductor of electricity, is neither positively or negatively charged. When your negatively charged finger approaches the metal doorknob the attraction becomes greater until...
12. 12. You may have noticed that you do not build upstatic electricity when you walk across a concretefloor. That is because some atoms hold on totheir electrons more tightly than others do.Examples of materials that are more apt togive up electrons are: fur, glass, humanhair, nylon, wool, silk.Examples of materials that are more apt tocapture electrons are: styrofoam, SaranWrap, polyurethane polyethylene (likeScotch Tape) polypropylene vinyl (PVC).
13. 13. Often when you take clothes from theclothes dryer, they seem to stick together.This is because some of the clothes havegained electrons by rubbing against otherclothing. The clothes losing electronsbecome positive and are then attractingthose pieces of clothing that have gainedextra electrons. Or, negativeclothes are attracted to the positiveclothes.
14. 14. The bottom line, once again: Objects with + and – charges attract one another. Objects with extra (-) charges push away away from each other. Objects with extra (+) charges push away away from each other.
15. 15. It’s like the poles of a magnet,“likes repel and opposites attract.”
16. 16. Now we will do an experiment todemonstrate what we have beendiscussing.
17. 17. Here we have two pieces of tape with the endswrapped around tooth picks.These two pieces of tape are marked with a “B”to show that they are on the bottom andsticking to the surface. B B
18. 18. Next we will stick two more pieces of tape ontop of the first two. They are marked with a“T” for top. B T B T
19. 19. Using your materials, set up the experiment bysticking (pressing) your “T” tape directly overthe (on top of) the “B” tape while it is stillsticking to the surface of your table.Now peel the top tapes off. They both had Belectrons stripped away when they were Bpeeled up. T TSo now the “T” tapes will have fewer electronsbecause of those they lost, but the samenumber of protons they started with, whichmakes them positively (+) charged.
20. 20. Now you are going to “test” your two “T” piecesof tape by holding them close to each other tosee if they repel or attract. Before you do, makea prediction.Next, peel up the “B” pieces of tape from yourtable and after making your prediction, testthem in the same way.Finally, test one of the “B” pieces of tape withone of the “T” pieces, but only after making aprediction whether they will repel or attract.
21. 21. Another vividdemonstrationof whathappens whenthe electricbalance of anobject is upsetis the Van deGraff generator.
22. 22. The Van DeGraff generatoris a device thatdemonstratesthe effects ofunbalancedcharges as canbe clearly seenhere.
23. 23. Van de Graff generators have several parts: a motor, a belt, tworollers, two "combs," and a metal sphere. The bottom roller ismade out of a material that loses electrons easily, and the upperout of a substances that readily captures electrons. As themotor turns, the rubber belt first goes over the bottom roller.A comb pulls electrons from the material on the bottom roller(which loses electrons easily) and transfers them to therubber belt. The belt then travels to the top roller. Thesecond comb near the top roller collects the electrons fromthe belt and stores them on the metal sphere. Themotor turns very fast, so the sphere quickly collects alot of electrons and becomes negatively charged and sodo you when you touch the dome. Touching a charged sphere is truly a "shocking" experience!
24. 24. When a person places their hand on the ball and themachine is turned on, electrons are transferred to andcollected on the person touching the silver ball. Why do you think this machine affects the hair of the children in the picture?
25. 25. Magnetism and electricity are related.If you run electricity through a wire, amagnetic field is set up around thewire-- the wire becomes a magnet aslong as electrons flow through it.Activity with circuit and compass.
26. 26. An electromagnet is a magnet that runs on electricity.An electromagnet works because an electric currentproduces a magnetic field.Unlike a permanent magnet, the strength of anelectromagnet can easily be changed by changingthe amount of electric current that flows through it. The poles of an electromagnet can be reversed by reversing the flow of electricity. If a wire carrying an electric current is formed into a series of loops, the magnetic field can be concentrated within the loops. The magnetic field can be strengthened even more by wrapping the wire around a core of soft iron.
27. 27. This business of an electric current running through acoil of wire and making a magnet opens all sorts ofpossibilities, like electric motors and electricgenerators.Any electric motor is all about magnets andmagnetism. A motor uses magnets to create motion.We will use this simpleBeakman motor forstudy. The armatureor rotor (in this casethe coil of copper wire)is an electromagnet.
28. 28. The ends of the copper wire in the coil make contactwith the pieces connected to the battery terminals.Current flows through the coil, making it into anelectromagnet.Since magnets attract, the coil is attracted to one poleof the ceramic magnet.Inertia causes the coil to continue around and whenthe coil nearly completes a spin, the process repeatsitself.
29. 29. Activity with electric motors
30. 30. We have just seen how electricity is used to makemotion, now we’ll see how motion is used to makeelectricity. This is a generator. It uses motion to generate electricity.
31. 31. A generator has As the shaft insidea long, coiled the generatorwire on a shaft turns, an electricsurrounded by a current isgiant magnet. produced in the wire.When theturbine An electricturns, the shaft generatorand rotor also convertsturn. mechanical, movin www.energyquest.ca.gov g energy into electrical energy.
32. 32. Consider the many things that we depend on daily that arepowered by electricity, and then realize our debt to itsdiscoverer. His name is Michael Faraday.The generator is based on the principle of"electromagnetic induction" discovered by MichaelFaraday, a British scientist in 1831 Mr. Faradaydiscovered that if an electric conductor, like a copperwire, is moved through a magnetic field, an electriccurrent will flow in the conductors.
33. 33. Activity with generator
34. 34. Next, we will examine the phenomenonthat keeps our feet on the ground: Gravity
35. 35. Gravity is a force.Gravity is a force that pulls.Every object has gravity.So every object pulls on every otherobject.The more mass an object has, the harderit pulls.
36. 36. We will use two hypothetical planets for ourexample. Both the blue and green planetsare pulling on each other. Which one pulls harder?
37. 37. This should help us see that the more mass an object has the stronger its gravity. Moon Earth The Earth obviously has more mass thanBut the Moon’s gravity is also pulling on the Moon and it pulls harder.the Earth. So hard that the oceans swell So much harder that theever it passes. in antoward the Moon where Moon is held Wecall this high tide. Earth as though by some orbit around the magically strong string.
38. 38. Let’s think about gravity on our favorite littleplanet, Earth.
39. 39. Gravity on Earth pulls everything towardits center.
40. 40. That’s why there isno top or bottomand no one fallsoff.
41. 41. If you dug a holeright through theEarth and fellin, how far wouldyou fall?
42. 42. You’d fall into the hole and shoot right past the center because you would be going so fast. As soon as you passed it, youwould be pulledback towards thecenter. So youwould bounce backand forth like abungie jumper tillyou finally stoppedat the center.
43. 43. On Earth it appears that not everything falls to the ground at the same rate.It seems to us that things with lessmass or weight e.g. feathers fallslower than things with more massor weight e.g. rocks. This is a misconception.As demonstrated by Galileo in the 1500’s, allobjects in a vacuum, fall at the same rateregardless of mass.Lighter objects on Earth fall slower due toour atmosphere which slows their descent.
44. 44. On the Moon, an astronaut dropped afeather and a hammer.Since there is no atmosphere on theMoon the feather and the hammer hitthe ground at the same time.
45. 45. Demo dropping book and paper.1st Drop a book and wadded up paper at the same time. (land together)2nd Drop a book with a sheet of paper touching the under side of the book. (land together)3rd Drop a book with a sheet of paper touching the top of the book. (land together)4th Drop a book and a sheet of paper separately but at the same time. (book lands before paper) Use previous slides to explain why.
46. 46. The weight of an object is a measure ofhow hard gravity pulls on it.However, the amount of gravity on eachplanet differs. The Moon has only one-sixthas much gravity as the earth.Consequently, on the moon you wouldweigh only one-sixth of what you weigh onEarth.
47. 47. This boy weighs60 pounds onEarth.
48. 48. On the Moon hewould only weigh10 pounds.
49. 49. Since each planet has adifferent amount ofgravity, this boy’sweight would changeeach time he went toanother planet.
50. 50. OnJupiter, thiswould be likecarrying anextra 100poundsaround onyour back allday.
51. 51. The farther an object is from the center ofa planet, the weaker the force of gravity.So, would this apple weigh more in someplace like Death Valley or on top of a veryhigh mountain?
52. 52. Weight of a one pound apple heading outto space. Apple Apple Apple Apple weighs weighs weighs weighs 1 pound 1/4 pound 1/9 pound 1/16 pound here here here here
53. 53. In a spaceship like the shuttle, you would beweightless. However this is not becauseyou are so far from Earth that there is nogravity. It is because the spaceship, beingpulled by gravity, is always falling frombeneath you.
54. 54. Both of these men are weightless, still they areboth being pulled by gravity. They have weightonly when gravity pushes them againstsomething like the floor or a scale.
55. 55. What causes gravity?Even the great At his timeSir Isaac Newton and stillcouldn’t answer today, whatthat one. causes gravity is a mystery.But it is a force that effects everything inthe universe.
56. 56. ForceandMotion
57. 57. Motion is the process of an object moving.An object’s motion changes when a force actsupon it.
58. 58. Newton’s Three Laws of Motion
59. 59. Newton’s First Law of Motion--Inertia• an object at rest stays at rest unlessacted on by another force• an object in motion stays in motionunless acted on by another force
60. 60. Motion is a relative term.All matter in the universe is moving all thetime, but the motion referred to in the firstlaw is a position change in relation tosurroundings.We live on the Earth which is rapidlyrotating and orbiting the Sun. But whenwe sit down we say we are at rest.
61. 61. When you are sitting in your seat in anairplane flying through the sky, you areat rest.But, if you get up and walk down theairplanes aisle, you are in motion.
62. 62. In order to understand the first law it is important to understanding balanced and unbalanced forces. If you hold a ball in your hand and keep it still, the ball is at rest. All the time the ball is held there, it is being acted upon by forces. The force of gravity is trying to pull the ball downward, while at the same time your hand is pushing against the ball to hold it up.The forces acting on the ball are balanced.
63. 63. Let the ball go, or move your hand upward,and the forces become unbalanced.The ball then changes from a state ofrest to a state of motion.
64. 64. If you are not inmotion rightnow, chances arethat you havebalanced forcesacting on you .
65. 65. Now let’s get back to discussing the firstpart of Newton 1st Law of Motion.The first part of this law seems prettyobvious—an object stays at rest until aforce acts upon it.
66. 66. A ball sitting onthe ground is atrest and when itis rolling orflying it is inmotion.
67. 67. Furthermore, aresting ball staysresting until a forceacts upon it—in thiscase a moving foot.
68. 68. The second part of this law is lessobvious—an object in motion stays inmotion until a force acts upon it.This is a difficult concept because in ourexperience things do slow down andstop, they don’t keep moving in a straightline and at the same speed.The reason, of course, is that there is aforce acting on those things. The force isusually friction, which we will study later.
69. 69. This second partof the first law ofmotion explainswhy we shouldwear seat belts.The car andperson are bothin motion andwhen the carstops abruptlythe person staysin motion flyingout of the car.
70. 70. Astronauts who “walk in space” are tethered to the shuttle or space station so they do not float off into space.Otherwise, when they push against the spacecraft, theywould start moving away from the ship and continuemoving out into space in a straight line until acted on byanother force.
71. 71. Activity with car and clay: place a gob of clay on atoy car, run the car into something fixed, car stopsclay flies forward. (inertia)Activity: stack of pattern blocks (or poker chips) and a bladewith which to strike the bottom one. Bottom one fliesaway, other stay (inertia).Another: place an index card on a beaker or cup, place a pennyon index card then thump the card (card flies away, pennydrops straight down (inertia).
72. 72. Newton’s First Law of Motion combined with the Law of Gravity explains why a planet or moon orbits another (and larger) object. The planet or moon is actually moving in a straight line that would carry it away from the larger object it is orbiting.At the same time, the force of gravity pulls the planet ormoon towards the larger object.As a result of the two balanced forces, the planet ormoon keeps falling into orbit around the larger object.
73. 73. Newton’s Second Law of MotionAn object’s acceleration depends directlyupon the net force acting upon theobject, and inversely upon the mass of theobject.As the force acting upon an object isincreased, the acceleration of the objectalso increases.As the mass of an object is increased, theacceleration of the object decreases.
74. 74. Acceleration is either a change in speed(speeding up or slowing down) or a changein direction. Same speed, same direction, this is not acceleration. or slowing Speeding up down, is acceleration. A change in direction is acceleration
75. 75. First:An object’s acceleration is directlyproportional to the force. Forexample, if you are pushing on anobject, causing it to accelerate, andthen you push, say, three timesharder, the acceleration will bethree times greater. If you push twiceas hard, it will accelerate twice asmuch.
76. 76. Second:This acceleration is inversely proportionalto the mass of the object. For example, ifyou are pushing equally on two objects, andone of the objects has five times more massthan the other, it will accelerate at one fifththe acceleration of the other. If it gainstwice that mass it will accelerate half asmuch.
77. 77. Sometimes a picture can say more thanwords. Let’s see. We have a large force and a small mass. The large force is applied to the small mass. The small mass accelerates rapidly.
78. 78. Or, in the other case: We have a small force and a large mass. The small force is applied to the large mass. The large mass accelerates slowly.
79. 79. A speeding bullet and a slow moving trainboth have tremendous force. The force of thebullet is a result of its incredible accelerationwhile the force of the train comes from itsgreat mass.
80. 80. A bowling ball has a lot more mass than a soccer ball.If a bowling ball and a soccer ball were bothdropped at the same time from the roof of a tallbuilding obviously, because it has moremass, the bowling ball would hit the groundwith greater force than the soccer ball.We know that gravity accelerates all objects atthe same rate, so both balls would hit theground at the same time.
81. 81. Therefore, the differences in force wouldbe caused by the different masses of thetwo balls.Newton stated this relationship in hissecond law, the force of an object isequal to its mass times itsacceleration.
82. 82. Force 50 N If the mass of an object doubles, you would need to exertForce 100 N twice the force to accelerate it at the same rate.
83. 83. When you plug in the numbers for force in the Notice that doubling the force by addingillustration above, (100 N) and mass (50 another dog would double the acceleration.kg), you find that the acceleration is 2 m/s2. Oppositely, doubling the mass to 100 kg would halve the acceleration to 2 m/s2.Right granted for use for noncommercial use How Stuff Works
84. 84. It is the force ofgravity thatcauses anobject to movedown a ramp orinclined plane.The more mass an object has the greater the force ofgravity pulling on it even in this situation.However, the acceleration of the objects be the same.They will move down the ramp at the same rateregardless of their mass.
85. 85. Experiment to demonstrate Newton’sSecond Law of Motion (balls ofdifferent masses)
86. 86. Newton’s Third Law of MotionFor every action, there is an equal andopposite reaction.
87. 87. The rider steps off the skateboard.In the Third Law, the stepping off theskateboard is called the action.The skateboard responds to that action bytraveling some distance in the oppositedirection. The skateboards oppositemotion is called a reaction.
88. 88. When you compare the distance traveledby the rider and the skateboard arecompared, it appears as if the skateboardhas had a much greater reaction than theaction of the rider. This is not the case.The reason the skateboard has traveledfarther is that it has less mass than therider—the Second Law of Motion.
89. 89. If twopeople, both onskateboards, push on one another(action), theymove away in theoppositedirection as thepush (reaction) .
90. 90. When this man onroller skates pusheson the car, the cardoesn’t movebecause it has greatmass but he whohas little mass rollsbackwards.
91. 91. When a gun fires, the bullet movesforward (action) causing the gun to recoil(reaction).
92. 92. When a balloon full of air issealed, the air pressure onboth inside and outside arebalanced, same pressure.When the balloon is not tiedthe air inside the balloonescapes and then the airpressure outside the balloonis greater than inside.As a result of the air moving out of the balloonin one direction, the balloon moves in theopposite direction—action, reaction.
93. 93. In both the balloon and rocket engine shownabove, gases rush downward (action) causingthe balloon and rocket to go up (reaction).
94. 94. Activity with balloon “rocket”
95. 95. Along with Newton’s Laws of Motion, we nowconsider Friction.Considered by some to be one of the basicforces, friction is the force that opposes motionwhen an object’s surface is in contact withother objects. Although we seldom think about the role it plays, friction is crucial to many things we do....often making our lives more difficult and often making it easier.
96. 96. For example, it is friction between theground and the sole of our shoes that makewalking possible and it is lack of friction thatmakes our feet slip on ice or highly polishedsurfaces.Without friction, the belts of machines wouldslip, nails and screws wouldn’t hold, wheelswould spin without making things move.At the same time friction wastes energy andcauses our machines to break down and towear out.
97. 97. Friction is the force that opposes motion.To move the blue bar over the orange bar, frictioncould be a problem.The greater the “load” the more “force” will beneeded to overcome “friction.” force
98. 98. The two major types of friction are:Sliding friction: The rubbing together of thesurface of a moving body with the materialover which it slides.Static friction: the force between twobodies in contact that opposes sliding.
99. 99. Sliding friction-can be easily demonstrated inthe classroom.Put both of your hands together and movethem back and forth. Push your hands togetherharder and move them faster. What do youexperience? Are your hands warming up? Doyou hear the sound of the hands movingagainst each other?Friction results from the surface of your handsmoving in opposite direction over each other.Because your hands are in motion this type offriction is known as sliding friction.
100. 100. Many teachers have dealt with the problem of moving the “big” box of new books when all the carts were already taken.Here itis ingraphicform. Sliding friction
101. 101. Sliding friction between the: the broom and the floor the foot and the floor the hand and the hat
102. 102. Now let’s look at static friction—the forcebetween two bodies in contact that tends tooppose sliding.In order to move something, you must firstovercome the force of static frictionbetween the object and the surface onwhich it is resting.
103. 103. Football players understand static friction well.When they first hit this blocking sled, it very muchresists moving (static friction). Once moving, the sled becomes somewhat easier to push as sliding friction becomes the main force resisting movement.
104. 104. If you haveevery pushed acar you haveexperiencedstatic friction.Initially youhave to pushreally hard toget the carmoving. That isstatic friction.Once you have the car rolling it is easier to keep thevehicle moving. That is sliding friction.
105. 105. This picture shows static friction, just before the block moves. This picture shows sliding friction, while the block is in motion.It takes more force to get the block moving—staticfiction than it does to keep it moving—sliding fiction.
106. 106. (Activity: Use scales to determinethe force (in newtons) required tomove a brick in basket.)
107. 107. The amount of friction encountered, eithersliding or static, will depend on two things:1. How smooth two surfaces arethat are touching.2. The weight of the moving bodyor the body you are trying to get tomove.
108. 108. Even though a surface may look verysmooth, friction occurs in part because nosurface is perfectly smooth.Rough surfaces have grooves andridges which catch on one another asthe two surfaces slide past each other.When two surfaces try to move past eachother these little bumps collide and slowdown the motion of the surfaces.
109. 109. The rougher a surface is, the more and biggerbumps it has--more friction. The smoother asurface is, the fewer and smaller bumps—lessfriction.For example if you slide a wooden block downa ramp it will be slowed by friction. If yousandpaper the block to make it smooth, theblock will be smoother and slide faster.If you cover the block in sand paper (making itrougher) the block moves even slowerbecause of the sandpaper’s rough surface.
110. 110. Even surfaces that are apparently smooth can berough at the microscopic level. Under amicroscope, no surface is really "smooth." No matter how smooth the surfaces may look to your eyes, there are many ridges and grooves.The ridges of each surface can get stuck in thegrooves of the other.
111. 111. Sandpaperviewedunder amicroscope
112. 112. Clothviewedunder amicroscope
113. 113. Surface of atile viewedunderamicroscope
114. 114. Paper viewed under a microscope
115. 115. Friction activity with different surfaces
116. 116. Once more, the amount of frictionencountered, either sliding or static, willdepend on two things:1. How smooth two surfaces arethat are touching.2. The weight (or mass) of themoving body or the body you aretrying to get to move.
117. 117. The more mass or weight an object has the morefriction it has. Therefore it will take more force toget it moving and more force to keep it moving. A dump truck has more mass than a Smart Car.
118. 118. The affect of weight on friction:If it takes 10 newtons of force to slide ablock with a weight of 50 newtons, it willtake 20 newtons of force to slide a block thatweighs 100 newtons:
119. 119. Very interesting!!!Friction does not depend on the amount of surfacearea in contact between an object and the ground, asdemonstrated in Example B.
120. 120. So, is friction good or bad?The answer, of course, is YES.Sometimes friction works against us andsometimes it works for us. It dependson the situation.
121. 121. How does friction works against us?Friction between the moving parts of anengine resists the engine’s motion andturns energy into heat, reducing the theefficiency of the machine and causing it towear out.Friction also makes it difficult to slide aheavy object, such as a refrigerator orbookcase across the floor.
122. 122. In others situations, friction is helpful.We would be unable to walk if there was nofriction between our shoes and the ground. It isthat friction that allows us to push off the groundwithout slipping.On a slick surface, suchas ice, shoes slip andslide instead ofgripping. This lack offriction, makes walkingdifficult.
123. 123. Friction allows the tires on our vehicles to gripand roll along the road without skidding.
124. 124. Activity with person walking on boardon dowels. Place a piece of plywood (could be about 3 x 3 ft.) over several aligned wooden dowels and have a students try to walk on the plywood. The board flies backward and the student stays put. (lack of friction)
125. 125. Friction between nails, screws and beamsprevents the nails and screws from sliding outstopping our buildings from collapsing.
126. 126. We want treadon our tires sowe can driveour cars and toprevent us fromslipping aroundon wet surfaces.
127. 127. We want tread ourfootwear so we cangain traction.
128. 128. Often however, we wish to reduce friction.Less friction makes it easier to movethings.Reducing the amount of friction in amachine increases the machine’sefficiency. Less friction means less energylost to heat, less noise and less wear andtear on the machine.
129. 129. People normally use three methods to reduce friction. The first method involves reducing the roughness of the surfaces in contact. For example, sanding materials lessens the amount of friction between the two surfaces when they slide against one
130. 130. The second method is to use smoothmaterials which create less friction.
131. 131. Or byputting asmoothsurfaceunder theroughersurface.
132. 132. The third way to reduce friction is often thebest way--replace static or sliding friction withrolling friction and/or add a lubricant.Rolling friction: Instead of sliding surfacestogether, you can place rollers betweenthem.Lubricant: By adding a thin layer of oil orgrease between two objects, you canreduce static or sliding friction and lessenwear on machines.
133. 133. Rolling frictionWhen a cylindrical or spherical body rollsover a surface, the force opposing themotion is called rolling friction. Addingrollers between two surfaces reducesfriction.
134. 134. Ball bearings arean example ofrolling friction.
135. 135. Friction can be reduced by adding a lubricant such as grease or oil between the two surfaces.Lubricants reduce friction by minimizingthe contact between rough surfaces. Thelubricant’s particles slide easily againsteach other resulting in far less friction.
136. 136. Activity with shaving cream as alubricant andAir Carts using air to separate surfacesand reduce friction.
137. 137. Lubricants decrease the amount of energy lost toheat and damage to machine surfaces.
138. 138. Oil Grease Two common lubricants
139. 139. That’s it for today...MAY THEFORCE BEWITH YOU!