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Lesson on simple machines and frictionThis introductory session is aimed for 6-7th graders learning physics and invites participation by studentsIntroduction of subject matter“Physics is a science which concerns itself with the fundamental laws of a material universe.” Questions for students’ discussionWhat is a science, what are the requirements of a science?What is meant by “fundamental.” What is meant by laws?” What are “fundamental laws?’ Do you know any fundamental laws?What is meant by “material?” What is non-material or the opposite of material?What is meant by the “universe?” Science is an organized body of knowledge arrived by: observation, recording of information, analysis of information, prediction, and verification. This is called the experimental process. Observation can mean many things. In physics observation usually means measurement. What is meant by measuring? What can we measure? What can’t we measure or what is difficult to measure? We can only measure three things or derivations from these three things. You are familiar with them. Questions for students: What do you think we can measure? Use your experience. It boils down to three items ultimately only two (to be later explained).We can measure force, distance, and time. Questions for students: What is meant by force? Give examples of horizontal and vertical forces. How do we measure force? How do we measure distance? How do we measure time? Teacher shows force gages, meter or yard stick, and clock. We need a standard to measure anything. We have to compare our results with the standard.Questions for students: What are the standards for measuring force, distance and time? So what do we need to make a measurement of force, distance, and time?We need a standard. What is a standard? We need an instrument calibrated to the standard. And we need a reference frame. What is a reference frame? A reference frame describes all the conditions when making measurements such as location, perhaps temperature, perhaps weather, perhaps motion, perhaps noise level, indoors or outdoors. Questions for students: Why is recording the reference frame important even before making measurements? WheelsThe first example of a force we shall discuss is friction. Friction is a restraining force or a resisting force arising from two surfaces that rub against each other. Questions for students: Give me examples of high friction and low friction. When do you want high friction and when do you want low friction? High friction: shoes, tires on the road, pen on paper, brake pads, Low friction: ice skating, skiing, auto engine and transmissionWe are going to discuss the frictional force between a brick and piece of wood and how the use of wheels can decrease friction. We have to discuss how we orient the brick on the surface. First we examine the brick. We have to decide in which orientation we should weigh the brick. Does it make a difference? Student weighs brick. About 20 newtons. Explain that 4.5 newtons = 1 lb. so brick weighs 4.44 lbsAnother student re-orients brick and weighs brick. Write down in notebooks. Observe and record the answer. Does orientation make a difference? Why not? Now set brick on plywood horizontally and pull. Read gage. Ans: About 10 newtons. Ask students what they expect if I re-orient brick. Another student pulls brick and reads gage. Ans. 10 newtons. I re-orient again and another student pulls brick and reads gage. Repeat experiment by pushing gage. Does pulling or pushing make a difference? Repeat by adding a second brick. What is the pull force? Is it doubled? Replace plywood with a smooth board band pull the brick without dowels.How does the answer compare with using the plywood?Now define coefficient of friction = pull force or push force divided by weight = u(1) Make calculation.What does coefficient of friction really mean?Now place wood dowels under brick and measure pull/push force= u(2) = weight / pull forceDo the same with the smooth board. Measure and record the result. What conclusions can you make from all these tests? Put brick on small dolly and pull dolly. Measure the force. Is the force less than using the dowels? Define term: Actual Mechanical Advantage (AMA) = pull force without dowels/pull force with dowelsAMA = Pull force w/o dowels / pull force w dowels = [weight X u(2) ]/ weight X u(1) = u(2) / u(1)What does AMA mean? Question for students: Suppose our brick weighs 1,000 newtons or 2,222 lbs (over a ton), how much force is required to pull it with and without dowels? How much can one person pull in lbs, in newtons? 1lb = 4.5 newtons. How many people are needed to pull a 1,000 newton brick using dowels? One person can pull 200 newtons (44.4 lbs). Answers: 1 person and 5 people.Thought experiments. Now imagine we change the reference frame. We pull at high altitude where gravity is smaller. What do you think will happen? We pull the brick on the moon. What do you think will happen? The efficiency of moving a brick = 100% because the entire force to move the brick is needed to overcome friction. If there were no friction, the brick could be moved with an impulse to overcome inertia. The IMA = 1 because every inch pushed = 1 inch moved. Use of Simple Machines can magnify human effort to lift and move material objects. Questions for students: Name six simple machines. Wheels for movingInclined Planes for liftingWedge for separating and splittingLever for liftingPulleys for liftingScrew jack for liftingInclined PlaneQuestions for students: Look at free body diagram. Which ancient people used inclined planes to lift large blocks of stone?Questions for students: Give examples of using inclined planes to raise or lower material.Ramps on Moving Vans, Tower of BabelLook at the free diagram from the slides. The sum of the vertical forces = 0. The sum of the horizontal forces = 0. When the forces in all directions = 0, it means that the body is not moving or moving with constant velocity. We call this equilibrium (not accelerating). [Newton’s 3rd law] For every action, there is an equal and opposite reaction. Teacher Explains acceleration by entering Mass Pike from zero speed. We are going to measure the force required to pull or push a brick up an inclined plane in our school reference frame.Set the ramp at 8.16 degrees, height = 4” and the hypotenuse = 28 “Now pull up the brick without dowels and with dowels and measure forces required using smooth and rough surfaces. What forces do we have to overcome to move the brick up an inclined plane without dowels? Measure force to move brick up the inclined plane without and with dowels. The pull force does two things. It elevates the brick and overcomes friction. The pull force is parallel to the plane, pulling the brick at other angles will increase the pull force. Try it!The frictional force or resistance is also parallel to the plane. Show free body diagram.Fraction of pull force to lift the weight = [height/ hypotenuse] X weight of brickMeasure height and hypotenuse, calculate fraction and force to overcome gravityFrictional force or resistance = fraction of weight perpendicular to plane X coef of frictionFrictional force = [length / hypotenuse] X coef of frictionAdd two forces:Pull force = weight of brick [height/hypotenuse + coef of friction X [length/hypotenuse] Substitute numbers for each item and compare with our measurement of pull forceCompute Actual Mechanical Advantage = AMA = weight/pull forceCompute new parameter, Ideal Mechanical Advantage (IMA) = distance brick moves/height moved = length of inclined plane/ height movedThe efficiency of a simple machine is a measure of the output work/ input work .Later you will learn (when discussing energy) that the efficiency of a simple machine = AMA/IMA. Using our AMA and IMA, compute efficiency = AMA/IMAPresence of friction reduces efficiency. Angle of reposeThe angle of repose is the angle between the length and the hypotenuse where the brick begins to fall backwards in the absence of a pull force. We already stated that: Pull force = weight of brick [height/hypotenuse + coef of friction X length/hypotenuse] Now set the pull force to = 0 and allow the weight to pull the brick downhill We get: weight [- height/hypotenuse + coef of friction X length/hypotenuse]Coefficient of friction = height/length for the brick to start to move backwardsRaise the height to where the brick starts to move backwards. Measure height and pare with coefficient of frictionWhen the car is powered, the maximum angle that the car can still move is also called the angle of repose.__________________________________________________________________________________LeverLevers are used to lift weights, rotate nuts, & open bottles. Question for Students: Give applications of levers. Give examples of when levers are needed.The pivot point is called the fulcrum.Place bucket of water on end of lever and raise the bucket.Measure the distance from the fulcrum to the bucket handle, distance = aMeasure the distance from the fulcrum to your hand, distance = b.Divide b/a. This is proportional to the distance your hand moves divided by the distance the weight moves. This is the Ideal Mechanical Advantage (IMA)Now use the force gage to measure the applied force to lift the weight.Measure the weight of the bucket of water.Divide the bucket weight by the force to lift the bucket. This is the actual Mechanical Advantage AMA).Now divide AMA/IMA to learn the efficiency. Levers demonstrate the Law of Torques.Has anyone removed a nut from a tire rim. It requires a torque wrench or a wrench 2 feet long. Has anyone tried to open a well-sealed bottle? Can you do it with only your hands? You may need a special bottle wrench. Torque about a point on the lever = force X distance from the fulcrum. In our case, weight X distance from the fulcrum = applied force X distance from the fulcrum.If the torques about the fulcrum are equal, the lever is in equilibrium. Equilibrium means no acceleration [zero velocity or constant velocity] PulleysQuestions for students: What is a pulley? What are some uses of pulleys? Do pulleys have friction? How can we measure pulley friction?Show slide for two and four pulley systems.In any pulley system carrying or moving a weight, there is rope tension. The rope tension = the pull force for any pulley system.How many people are required to lift a stone weighing 1,000 newtons or 2,222 lbs using a 4 pulley system? IMA = 4. Ans. 5 people.What is meant by tension in the pulley rope? Why is tension important to know? Questions for students: Give examples of excessive tension or insufficient tension.VOCABULARY LISTPhysicsScienceFundamentalLawsMaterialForceMeasurementsStandardReference FrameFrictionReactive ForceNewtonCoefficient of frictionActual Mechanical AdvantageIdeal Mechanical AdvantageSimple MachinesFree Body DiagramVertical ForceHorizontal ForceAngle of ReposeLeverFulcrumHypotenuseTorqueEfficiencyPulleyTension ................
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