Topic Force PHYSICS 231 Current Assignments Homework Set 2 due this Thursday Jan 27 11 pm Reading for next week Chapters 10 1 6 10 10 8 3 2 1 11 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 2 1 11 Physics 231 Spring 2011 3 Clicker question A ball is thrown into the air vertically At its highest point the acceleration is A 0 B 0 C 0 2 1 11 Physics 231 Spring 2011 4 Clicker question A ball is thrown into the air vertically As it has left the throwing hand and is moving upward what describes best what is going on A An upward force keeps the ball moving upward B There are no forces acting on the ball C A downward force slows the ball down 2 1 11 Physics 231 Spring 2011 5 Reading Quiz 1 A net force is A the sum of the magnitudes of all the forces acting on an object B the difference between two forces that are acting on an object C the vector sum of all the forces acting on an object D the force with the largest magnitude acting on an object Slide 4 7 Reading Quiz 2 Which of the following is NOT one of the steps used to identify the forces acting on an object A Name and label each force the object exerts on the environment B Name and label each contact force acting on the object C Draw a picture of the situation D Identify the system and the environment E Name and label each long range force acting on the object Slide 4 9 Force and acceleration Force causes acceleration in the direction of the Force a F m 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 2 1 11 Physics 231 Spring 2011 8 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 2 1 11 Physics 231 Spring 2011 9 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 same as force pushing on surface just enough to cancel all forces perpendicular to surface n w 2 1 11 Physics 231 Spring 2011 n w mg w mg 10 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 2 1 11 Physics 231 Spring 2011 11 Friction For coefficients k s see for example http www engineershandbook com Tables frictioncoefficients htm 2 1 11 Physics 231 Spring 2011 12 Force and motion First law is a special case if Fnet 0 then a 0 so motion with constant velocity 2 1 11 Physics 231 Spring 2011 13 Clicker Quiz If no forces at all act on a moving object A it immediately stops B it slows down after some time C it keeps moving with constant speed D its speed increases 2 1 11 Physics 231 Spring 2011 14 Clicker question Elevator accelerates upward goes up with increasing speed What can you say about the normal force n pushing from the floor on the feet of a person and the persons weight w mg A n mg B n mg C n mg 2 1 11 Physics 231 Spring 2011 15 Clicker question Elevator moves downward with constant speed What can you say about the normal force n pushing from the floor on the feet of a person and the persons weight w mg A n mg B n mg C n mg 2 1 11 Physics 231 Spring 2011 16
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