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exam2studyguide

exam2studyguide - 6.R eview Summa ry 6 F r iction W hen a...

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6. 6 Review & Summary

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Friction When a force tends to slide a body along a surface, a frictional force from the surface acts on the body. The frictional force is parallel to the surface and directed so as to oppose the sliding. It is due to bonding between the body and the surface. If the body does not slide, the frictional force is a static frictional force . If there is sliding, the frictional force is a kinetic frictional force . 1. If a body does not move, the static frictional force and the component of parallel to the surface are equal in magnitude, and is directed opposite that component. If the component increases, f s also increases. 2. The magnitude of has a maximum value given by (6-1) where is the coefficient of static friction and F N is the magnitude of the normal force. If the component of parallel to the surface exceeds , the body slides on the surface. 3. If the body begins to slide on the surface, the magnitude of the frictional force rapidly decreases to a constant value f k given by (6-2) where is the coefficient of kinetic friction.
Drag Force When there is relative motion between air (or some other fluid) and a body, the body experiences a drag force that opposes the relative motion and points in the direction in which the fluid flows relative to the body. The magnitude of is related to the relative speed v by an experimentally determined drag coefficient C according to (6-14) where is the fluid density (mass per unit volume) and A is the effective cross-sectional area of the body (the area of a cross section taken perpendicular to the relative velocity ). Terminal Speed When a blunt object has fallen far enough through air, the magnitudes of the drag force and the gravitational force on the body become equal. The body then falls at a constant terminal speed v t given by (6-16) Uniform Circular Motion If a particle moves in a circle or a circular arc of radius R at constant speed v , the particle is said to be in uniform circular motion. It then has a centripetal acceleration with magnitude given by (6-17) This acceleration is due to a net centripetal force on the particle, with magnitude given by

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(6-18) where m is the particle’s mass. The vector quantities and are directed toward the center of curvature of the particle’s path. 7.10 Review & Summary Kinetic Energy The kinetic energy K associated with the motion of a particle of mass m and speed v , where v is well below the speed of light, is (7-1) Work Work W is energy transferred to or from an object via a force acting on the object. Energy transferred to the object is positive work, and from the object, negative work. Work Done by a Constant Force The work done on a particle by a constant force during displacement is (7-7, 7-8) in which is the constant angle between the directions of and . Only the component of that is along the displacement can do work on the object. When two or more forces act on an object, their net work is the sum of the individual works done by the forces, which is also equal to the work that would be done on the object by the net force of those forces.
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