Rev. 2 REVIEW SHEET #3: Forces; Newton’s Laws of Motion Acceleration of a body is caused by a non-zero net force acting on it: Newton’s First Law – A body at rest will remain at rest and a body in motion will continue to move with constant velocity unless it is acted on by a non-zero net force. ( 0 if 0) ext a F = = ∑ r r Forces are vectors. We discussed a number of common forces: (a) weight – magnitude is equal to mg and direction is always down to earth’s center; (b) normal force – direction is always perpendicular to the surface that exerts it; (c) tension – direction is always along the cable or string, away from the object; (d) frictional force – direction is always opposite to motion or intended motion; Static friction: F f ≤μ s *N Sliding friction: Ff=μ k *N The net force is the vector sum of all the external forces that are acting on an object: net ext F F = ∑ r r Newton’s Second Law – If a net external force acts on a body, the body accelerates. The direction of the acceleration is the same as the direction of the net force.
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This note was uploaded on 09/11/2011 for the course PHYS 0174 taught by Professor Kohler during the Spring '08 term at Pittsburgh.