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PS11 Static and Kinetic Friction

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85 views22 pages

PS11 Static and Kinetic Friction

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molemoshasha
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STATIC AND KINETIC FRICTION FRICTIONAL FORCE | Frictional force (f) is a resistance force that originates when two surfaces are in contact. Friction always opposes the relative motion between two surfaces. In contrast to the normal force, which is always exerted perpendicular to the surface, frictional force always operates next to or parallel to the surface, and in the opposite | rection of the object's motion. Frictional force (f) is the force that opposes the motion of an object and acts parallel to the surface that the object is in contact with. » Frictional force is not dependent on the size of the contact’ area between the surface areas, This statement can be tested with a wooden block and a newton spring balance. w First we place the block on the broad side and determine the expansion of the spring at the moment that the block is brought into motion. Then we turn the block onto its narrow side and do the same. If the sides of the block have the same finish, the expansion of the spring (spring balance reading) will be the same in both cases where the block starts to move. The magnitude of the frictional force is highly dependent on the nature of the two surfaces which are in contact with each other. The type of material and the finish have a great influence. Highly polished surfaces usually experience less friction than rough surfaces. In a motorcar engine or other machine, friction is undesirable and lubricants are used to minimize it as much as possible. On the other hand, maximum friction is required between a road surface and a car's tyre. One would not be able to walk if there were no friction between one’s foot sole and the floor. There are two types of friction, namely static friction (f,) and kine friction (j,). STATIC FRICTION Static friction (f,) is the frictional force exerted by one surface on another when there is no motion. Static friction is therefore the frictional force that must be overcome to move a static object. The following Figure =xplains the term static fri ‘No motion Figure 4 Figure 2 Figure 3 Figure 1 shows a large crate at rest on a floor. The crate is in static equilibrium because the weight and the normal force balance each other. No applied force (pulling or push force) is exerted on the crate. As long as there is no effort ro move the crate, there will be no static friction. iin Figure 2 a person is exerting a small horizontal force (Fapptica) on the crate. The crate is still not moving. The static frictional force (f,) equals the applied force (Fappiiod) in magnitude, but in the opposite direction, ln Figure 3 the applied force (Fappiiea) is slightly increased. As the the applied force (Fyppiiea) is increased, the static frictional force also increases and cancels the applied force. ln Figure 4 the applied force (Fapprioa) Is still increased up to a point where the crate breaks away from the surface aind starts moving. The magnitude of the applied force just before this point is reached, is called the maximum sitatic friction (or maximum static frictional force) and is denoted aS fijms.y Maximum static friction fijmqq, 8 the static friction just before an object The static frictional force increases to a maximUM finoxy a8 the forces which attempt to bring the object into motion increase ‘An object remains at rest as ong aS f, < fmayy | + The object starts moving when f, = fiasy A static frictional force where f, > fymax Is not physically possible. | Calculation of the maximum static frictional force (fmay) | The maximum static frictional force f,...) exerted on an object, depends on two factors, namely: | + The normal force (IN) exerted on the object by a surface. The greater the weight of the object, the greater the normal force exerted on the object by the surface (the harder the surfaces press against each other). A greate* applied force is now needed to bring the object in motion, Fuiman oN | + The types of surfaces (characteristics of the two surfaces) which are in contact with each other. The structure | and composition of materials determines the magnitude of frictional force exerted on another surface. A criterio® of the characteristics of the two surfaces and the type of materials in contact with each other, is indicated by the static friction coefficient (j,) of the material concerned. Ue, the static friction coefficient (1 is the Greek character *mu”) is a constant of proportionality and depends Cn the characteristics of the surfaces and types of materials which are in contact with each other. The value Of ps fluctuates from zero (surface without friction) to a maximum value (maximum static friction). The greater the value of ps , the coarser the surface and the more difficult it is to bring the object in motion. | Thus it follows that: HN Where: figs = Maximum static frictional force in N (newton) | Ii, = static friction coefficient N= normal force in N (newton) Because both fy... and N are forces measured in N (newton), NN is cancelled out so that only a number remains therefore j1, has No unit, The equation fiymoq= HeN only refers to the magnitudes of f.,...) and N, and not to the | directions. The two forces ji...) and N are perpendicular (90 °) to each other. The value of the static frictional force also fluctuates from zero to maximum, which can be calculated from the | equation fijmon HaN The frictional force f, (which is exerted while the object is at rest) < fjmay (ust before the | object starts moving): ThUS fuga) < Hal ‘The static friction coefficient (js) indicates the relation between the maximum static friction force and the normal force forthe two surfaces.. TT) The greater the maximum static frictional force (f,jy.,)) in relation to the normal force (NN), the greater and the more difficulty the two surfaces will move against each other. A smaller static friction coefficient (ti,) indicates surfaces | moving against each other with ease. KINETIC FRICTION : f, {Kinetic friction (f s) is the frictional force a surface exerts on an object when the object is | moving across the surface | ‘As soon as an object starts moving across a surface, static friction is not in question any more. The surface then exerts a kinetic friction on the object. The kinetic friction is less than the static friction, in other words. fi< Sums Calculation of kinetic friction (f) The kinetic friction (f,) depends on: + The normal force of the surface on the object. The normal force indicates how hard the surfaces press against each other. The harder the surfaces press against each other, the greater fis. Aan + The types of surfaces (characteristics of the two surfaces) in contact with each other. A criterion of the characteristics of the two surfaces and the type of 5 a friction coefficient (1,) of the material rs tei in contact with each other, is indicated by the kin ws it follows that: kinetic friction coefficient | 'N = normal force in N (newton) | kN only refers to the magnitudes of j,and N, and not to the directions. The two forces are perpendicular (90 °) to each other. The Kinetic friction coefficient (11x) indicates the relation between kinetic friction | force and to the normal force of the two surfaces in contact with each other. | Ji Mey FRICTION COEFFICIENTS Friction coefficients usually have values smaller than 1. Both static and kinetic friction coefficients are dependent on the characteristics (types) of the two surfaces in contact. Lubrication smooths surfaces and thus reduces the friction coefficient. | | | Because ji, is smaller than fijmsxy Hk Will also be smaller than 1, for the two surfaces in contact. (11, > Hy). | * Static friction coefficient We have seen that the maximum static friction is directly proportional to the normal force, fijmaa) % N. From thi follows thatthe relation is uu.» conatan "the two Surfaces involved. This constant i called the static fiction coefficient. The static friction coefficient (u,) is the relation Sumaa_ of the two surfaces involved fi z Sun = constant = uy fk ‘Graalent of graph = coofcint of Thus, p15 = “HE and fimaxy = Hel sF state loser € = Abeta The gradient of a graph of maximum static friction as opposed 2 ¥s=—AN to normal force, supplies the static friction coefficient: & a é For an object at rest the friction force is less than the § maximum static friction, therefore: E S

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