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Friction Introduction

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Physics, Friction,
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   Interaction of two objects in contact with each other with resulting forces is called contact force. The normal force, N, is a contact force that acts perpendicular to the common surface of contact and friction force, f, is a contact force that acts parallel to the common surface of contact. In previous experiments, Newton's laws were studied in an ideal setting wherein friction and air resistance were ignored. However, friction is something that cannot be ignored and must be taken into account for most practical situations. [1] The experiment aims to measure the coefficients of static and kinetic friction between two surfaces and measure the critical angles. Frictional forces act between two surfaces and oppose their relative motion.[2] There are two types of friction: static and kinetic. Kinetic friction is the friction between surfaces in relative motion. Experimentally, it is observed that the force of kinetic friction is proportional to the normal force acting between the surfaces [2] and is given as        (Eq. 1) where N is the magnitude of the normal force and    is called the coefficient of kinetic friction.[3] The coefficient of kinetic friction is a dimensionless quantity (no units) that depends on the properties of the two surfaces.[2] Static friction is the friction between surfaces that are at rest with respect to each other. Experimentally, is it found that the maximum value for the static frictional force is proportional to the normal force between the two surfaces [2] and is given as        (Eq. 2) The coefficient of static friction is greater than the coefficient of kinetic friction [3] because more molecular bonds are able to form, when objects are at rest with one another, making the object harder to move and so greater force is needed to start motion when compared to the case of the kinetic friction.[2] Graphically, this is shown in Figure 1: As you increase the force, the static friction force increases linearly until the applied force F equals sFN. After this point the object breaks away and the friction force falls to the kinetic friction value. [2] Figure 1  Relationship between frictional force and applied force  References [1] Giancoli, Douglas C. Physics: Principles with Applications 6th Ed. 2005. Pearson Education, Inc. United States of America [2] Experiment 6: Friction. Data Retrieved from http://science.iit.edu/sites/science/files/elements/phy/pdfs/2013_physics_123_friction_lab.pdf last 10/19/2014 [3]Chan, Kelvin. experiment 01: Force of Friction: Lab Manual. Data retrieved from http://www.mssch.edu.hk/phy/lab/TAS%20Lab%20Manual%20%26%20Report%2001%20_Force%20of %20Friction_.pdf last 10/19/2014
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