Peter Jeschofnig, Ph.D.
To provide an understanding of the concept of friction, and
To calculate the coefficient of friction of an object by two methods.
Qty Item Description:
Ramp board: 3 - 4 feet long, 10 cm wide
Can of soft drink or item of similar weight
Friction block set-PK
Tape measure, 1.5-m
Tape measure, 3-m
Discussion and Review
Whenever a body slides along another body a resisting force is called into play that is known as friction. This is a very important force and serves many useful purposes. A person could not walk without friction, nor could a car propel itself along a highway without the friction between the tires and the road surface. On the other hand, friction is very wasteful. It reduces the efficiency of machines because work must be done to overcome it and this energy is wasted as heat. The purpose of this experiment is to study the laws of friction and to determine the coefficient of friction between two surfaces. THEORY
Friction is the resisting force 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 but not on the area of contact or on the speed of the motion. We find 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 just the weight of the object. However, if the object is on an incline then it is not equal to the weight but is calculated by N= mg cos θ. The constant of proportionality is called the coefficient of friction, µ. When the contacting surfaces are actually sliding one over the other the force of friction is given by
Ffr = µk FN
where Ffr is the force of friction and is directed parallel to the surfaces and opposite to the direction of motion. FN is the normal force and µk is the coefficient of kinetic friction. The subscript k stands for kinetic, meaning that µk is the coefficient that applies when the surfaces are moving one with respect to the other. µk is therefore more precisely called the coefficient of kinetic or sliding friction. Note carefully 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 used 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 see in your later study of
physics 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 constant µk is to have one of the surfaces in the form of a plane placed horizontally with a pulley fastened at one end. The other surface is the bottom face of a block that rests on the plane and to which is attached a weighted cord that passes over the pulley. The weights are varied until the block moves at constant speed after having been started with a slight push. Since there is no acceleration, the net force on the block 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...
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