Friction Lab Essay

Discussion and ReviewWhenever a remains slides along another eubstance a resisting beat back is called into play that is known as clangoring. This is a precise important personnel department and serves many useful purposes. A person could not walk without friction, nor could a political machine propel itself along a highway without the friction between the tires and the road issue. On the other hand, friction is very wasteful. It reduces the efficiency of machines because work must be d genius to overcome it and this energy is wasted as heat. The purpose of this test is to study the laws of friction and to determine the coefficient of friction between two surfaces. THEORY attrition is the resisting military strength encountered when one surface slides over another. This force acts along the tangent to the surfaces in contact. The force necessary to overcome friction depends on the nature of the materials in contact, on their roughness or smoothness, and on the normal force provided not on the area of contact or on the speed of the motion. We encounter experimentally that the force of friction is directly proportional to the normal force. When an object is sitting on a horizontal surface the normal force is but the weight of the object. However, if the object is on an incline then it is not advert to the weight but is calculated by N= mg cos . The ceaseless of proportionality is called the coefficient of friction, . When the contacting surfaces are actually sliding one over the other the force of friction is coursen byEquation 1Ffr = k FNwhere Ffr is the force of friction and is directed parallel to the surfaces and opposite to the fashion of motion. FN is the normal force and k is the coefficient of kinetic friction. The subscript k stands for kinetic, meaning that k is the coefficient that applies when the surfaces are goone with respect to the other. k is therefore more precisely called the coefficient of kinetic or sliding friction. Note ca refully that Ffris always directed opposite to the direction of motion. This means that if you reverse the direction of sliding, the frictional force reverses too. In short, friction is always against you. Friction is called a non-conservative force because energy must be apply to overcome it no matter which way you go. This is in contrast to what is called a conservative force such as gravity, which is against you on the way up but with you on the way down.Thus, the energy expended in lifting an object may be regained when the object descends. Yet, the energy used to overcome friction is dissipated, which means it is lost or made unavailable as heat. As you will follow in your later study ofphysics the distinction between conservative and non-conservative forces is a very important one that is fundamental to our concepts of heat and energy.A method of checking the proportionality of Ffr, and FNand of determining the proportionality changeless k is to have one of the surfaces in the variety of a canvas placed horizontally with a pulley fastened at one end. The other surface is the bottom face of a overindulge that rests on the skim off and to which is attached a weighted cord that passes over the pulley. The weights are varied until the gag rule moves at constant speed after having been drop deaded with a slight push. Since there is no acceleration, the net force on the catch is zero, which means that the frictional force is equal to the tension in the cord.This tension, in turn, is equal to the total weight attached to the cords end. The normal force between the two surfaces is equal to the weight of the block and can be increase by placing weights on top of the block. Thus, corresponding values of Ffr,and FN can be found, and plotting them will show whether Ffrand FN are indeed proportional. The slope of this graph gives k. When a body lies at rest on a surface and an attempt is made to push it, the pushing force is unlike by a frictional force. A s long as the pushing force is not strong enough to start the body moving, the body remains in rest.This means that the frictional force automatically adjusts itself to be equal to the pushing force and thus to just be enough to balance it. However, there is a threshold value of the pushing force beyond which larger values will cause the body to break away and slide. Weconclude that in the quiet sideslip where a body is at rest the frictional force automatically adjusts itself to keep the body at rest up to a certain maximum. But if static equilibrium demands a frictional force larger than this maximum, static equilibrium conditions will cease to exist because this force is not available and the body will start to move. This situation may be expressed in equation form asEquation 2Ffr sFN or Ffr max = sFNWhere Ffris the frictional force in the static campaign, Ffr max is the maximum value this force can assume and sis the coefficient of static friction. We find that sis slightly larger than k. This means that a somewhat larger force is needed to break a body away and start it sliding than is needed to keep it sliding at constant speed once it is in motion. This is why a slight push is necessary to get the block started for the measurement of k.One way of investigating the case of static friction is to observe the so-called limiting fish of repose. This is defined as the maximum angle to which an lean plane may be tilted before a block placed on the plane just starts to slide. The arrangement is illustrated in Figure 1 above. The block has weight W whose function Wcos (where is the plane angle) is perpendicular to the plane and is thus equal to the normal force, FN. The component Wsin is parallel to the plane and constitutes the force urging the block to slide down the plane. It is opposed by the frictional force Ffr, As long as the block remains at rest, Ffr must be equal to W sin . If the plane is tipped up until at some value max the block just start s to slide, we haveEquation 3ButHenceOrThus, if the plane is gradually tipped up until the block just breaks away and the plane angle is then measured, the coefficient of static friction is equal to the tangent of this angle, which is called the limiting angle of repose. It is interesting to note that W cancelled out in the derivation of Equation 3 so that the weight of the block doesnt matter.PROCEDUREThis experiment requires you to record measurements in Newtons. Remember that in SI units the unit of force is called the Newton (N). One Newton is the force required to impart an acceleration of 1m/s2 to a mass of 1 kg. Thus 1 N = 1 kg.m/s2. You can convert any kg-mass to Newtons by multiplying the kg-weight by 9.8 m/s2, i.e., 100 g = 0.1 kg = 0.1 x 9.8 = .98 N. 1.Determining force of kinetic or sliding friction and static friction a. The wooden blocks provided in the LabPaq are too light to give good readings so you need to put some weight onthem, such as a full soft drink can. Weig h the apparent(a) wood block and the object used on top of the block. Record the combined weight in grams and Newtons.b. Place the ramp board you provided horizontally on a table. If necessary tape it down at the ends with masking tape to keep if from sliding.c. Begin the experiment by setting the block and its weight on the board with its largest surface in contact with the surface of the board. Connect the blocks hook to the 500-g spring weighing machine. d. Using the spring scale, slowly pull the block lengthwise along the horizontal board. When the block is moving with constant speed, note the force indicated on the scale and record. This is the approximate kinetic or sliding frictional force. Repeat two more times.e. While carefully watching the spring scale, start the block from rest. When the block just starts to move, note the force indicated on the scale and record. You should notice that this requires more force. This force isapproximately equal to the static frictional force. Repeat two more times.Determining coefficient of static friction using an inclined surfacea. Place the plain block with its largest surface in contact on the board while the board is lying flat.b. Slowly raise one end of the board until the block just breaks away and starts to slide down. Be very careful to movethe plane slowly and smoothly so as to get a precise value of the angle withthe horizontal at which the block just breaks away. This is the limiting angle of repose max. Measure it with a protractor (see moving-picture show that follows for an alternate way of measuring the angle) and record the result. You may also want to measure the base and the height of the triangle formed by the board, the support, and the floor or table. The height divided by the length of the base equals the coefficient of static friction.Rememberc. Perform two more trials. These trials should be independent. This means that in each case the plane should bereturned to the horizontal, the bloc k placed on it, and the plane carefully moved up until the limiting angle of repose is reached.DATA TABLE 6HeightBase Length maxsTrial 1Trial 2Trial 3AverageCalculations1. Using the mass of the block and the clean force of kinetic friction from Data put off 1, calculate the coefficient of kinetic friction from Equation 12. Using the mass of the block and the average force of kinetic friction from Data Table 2, calculate the coefficient of kinetic friction for the woodblock sliding on its side. Record your result and see how it compares with the value of kobtained from Data Table 1.3. From the data in Data Table 3, 4 & 5 compute the coefficient of static friction, sfor, the glass surface on wood, the sandpapered surface on wood, and wood on carpet, etc from each of your three trials. Calculate an average value of s.Record your results in your own data sheets.4.From the data obtained in Data Table 6 calculate sfor wood on wood from each of your three trials.5.Calculate an average va lue of s. Record your result on the data sheet.QuestionsA. How does the coefficient of static friction compare with the coefficient of kinetic friction for the same surfaces and areas?B. Why is it important to reduce friction during the operation of machinery? C. How does grease or oil affect the coefficient of friction?