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Unformatted text preview: Lab # 7 Coefficient of Kinetic Friction And Drag Force Introduction : The objective of this experiment is to investigate kinetic friction and drag force. We will measure the coefficient of kinetic friction for a wood block sliding on a plane. We will demonstrate that the friction force is independent of the area of contact between the block and the plane and that the force is proportional to the normal force acting on the block. We will do this by observing what magnitude force is necessary to keep an object in motion at a constant speed. To investigate the drag force, we will measure the terminal speed of free falling coffee filters in air. Figure 1: Adjustable incline plane shown with block (broad side down), mass hanger, masses and level . Note that the protractor is part of the adjustable incline plane apparatus. Equipment: Wooden blocks, Adjustable inclined plane with pulley and protractor, weight hanger and set of weights, platform balance and level, coffee filters and motion detector. Theory: Friction forces arise whenever objects with surfaces in contact move, or try to move, relative to one another. Frictionlike drag forces arise if a body moves through a fluid such as air or water. These forces are actually electromagnetic in nature, arising from the electrostatic interactions of the atoms and molecules of the two interacting objects, i.e. the two surfaces of the block and the inclined surface in figure 1 or in the case of a falling coffee filter in figure 4, its surface and the air molecules. Frictional forces separate into two distinct classes, static and kinetic friction. In the static case, the objects are at rest relative to one another but would be in motion if it were not for the force of static friction (that is, if one could reduce the static frictional force between the two surfaces they would begin to slide across one another). The force of static friction can increase from zero up to some maximum value Fs max before any motion occurs. When the object is just on the verge of sliding the force of static friction is at its maximum value, N s s F F = max (1) where F N is the magnitude of the normal force acting on the object and s is the coefficient of static friction. Its value depends on the properties of the two surfaces in contact, but not their surface areas. Once an object is in motion the properties of the force of friction acting between the two surfaces changes. The frictional force that acts on an object in motion is called kinetic friction....
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