Rev. 2REVIEW SHEET #3: Forces; Newton’s Laws of MotionAcceleration 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 unlessit is acted on by a non-zero net force. (0 if0)extaF==∑rrForces 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: Ff≤μs*NSliding friction: Ff=μk*NThe net force is the vectorsum of all the external forces that are acting on an object: netextFF=∑rrNewton’s Second Law– If a net external force acts on a body, the body accelerates.
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