Topic Force PHYSICS 231 Current Assignments Homework Set 2 due this Thursday Jan 27 11 pm Reading Chapters 10 1 6 10 10 8 3 Away on travel Monday Tuesday 1 31 and 2 1 Lecture will be held by substitute 1 24 2011 Physics 231 Spring 2011 2 Key Concepts Force Free body diagram all forces from the environment that act on an object Net Force Net Force causes acceleration of the object a F m No net force means constant velocity Special forces know magnitude and direction Tension in a string magnitude depends how hard you pull Normal force Friction static and kinetic Gravity on the surface of the earth Newton s 3rd law concerning forces between 2 objects 1 24 2011 Physics 231 Spring 2011 3 Force and acceleration Force causes acceleration in the direction of the Force Often many forces act on an object simultaneously The vector sum of all forces acting on an object from the environment is the net force Without net force a 0 and therefore constant velocity If something accelerates there must a corresponding net force be acting on the object 1 24 2011 Physics 231 Spring 2011 4 Gravity On the surface of the earth gravity acts on objects and exerts a force on them without touching Direction down center of the earth Magnitude w mg with g 9 81 m s2 this force is also called weight w 1 24 2011 Physics 231 Spring 2011 5 Normal force n An object pushing on a surface experiences a pushback force from that surface like a compressed spring Direction perpendicular to the surface Magnitude equals the magnitude of the force with which the object pushes onto the surface n w 1 24 2011 Physics 231 Spring 2011 n w mg w mg 6 Friction A surface touching a surface of an object exerts a friction force onto the object Direction parallel to the surface in the direction that opposes the motion that would occur without friction Magnitude Static friction force surfaces are not sliding has the magnitude needed to prevent motion Maximum static friction force fsmax n s Kinetic friction force surfaces sliding has the magnitude fk n k 1 24 2011 Physics 231 Spring 2011 7 Friction For coefficients k s see for example http www engineershandbook com Table s frictioncoefficients htm 1 24 2011 Physics 231 Spring 2011 8 Force and motion First law is a special case if Fnet 0 then a 0 so motion with constant velocity 1 24 2011 Physics 231 Spring 2011 9
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