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Unit 3: FORCE A. A force is that which affects the motion of a body. B. All known physical interactions of matter occur through four fundamental forces: Gravitation, Electromagnetism, Strong Nuclear Force and Weak Nuclear Force. 1. The most pervasive force is gravitation, in that every particle of matter attracts every other particle. C. Newton’s Second Law: The acceleration of a body is directly proportional to the resultant external force acting on the body, is inversely proportional to the mass of the body, and has the same direction as the resultant force. D. Newton’s Third Law: (Law of Interaction) If one body exerts a force on another body, then the second body must exert a force equal in magnitude but opposite in direction on the first body. E. Newton’s Universal Law of Gravitation: Every body in the universe attracts every other body with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. 3.2 Buoyant Force A. Archimedes Principle: Any object submerged or floating in a fluid is buoyed upward by a force equivalent to the weight of the fluid it displaces. 3.3 Friction A. The friction forces between two objects depends on the materials from which they are constructed and the force with which they are pressed together. 1. Contact between irregularities on the surfaces of objects obstructs motion. 2. Kinetic (sliding) friction is less than static (at rest) friction. 3. Static friction is a force tangential to the surfaces, exerted by one surface on another when the two surfaces are not sliding past each other. 4. Kinetic friction is similar, but occurs only when the two surfaces are sliding past each other. 3.4 Torque A. A torque is a force applied at a location on a body and in a direction necessary to cause rotation of that body. 1. “torquere” = twist 2. The torque about a given axis or fulcrum is the product of the force (F) and the lever arm (L). This product is called the moment of the force. The lever arm is the perpendicular distance from the axis of rotation to the line of force B. A simple machine such as a lever does not change the amount of work you must do, but it allows you to do the work using a smaller force. C. In a first class lever, the fulcrum is always between the effort and the resistance. 1. First class levers may be used to gain speed, force or change direction. D. In a second class lever, the resistance is always between the fulcrum and the effort. 1. Second class levers may be used to gain force. E. In a third class lever, the effort is always between the fulcrum and the resistance. 1. A third class lever can only be used to gain speed.