Topic Momentum PHYSICS 231 Current Assignments Set 4 will be due this Thursday Feb 10 11 pm Set 5 open but due week after exam Midterm exam on Thursday 2 17 2 28 11 Physics 231 Spring 2011 2 Reading Quiz What is Impulse A B C D a force that is applied at a random time a force that is applied very suddenly Force multiplied with the time its applied the interval of time that a force lasts 2 28 11 Physics 231 Spring 2011 3 Reading Quiz What is momentum A Mass times speed B Mass times velocity C Mass of a moving object D Mass times acceleration 2 28 11 Physics 231 Spring 2011 4 Key Concepts Momentum Vector Change of momentum is caused by a force applied for a time Impulse Collision Momentum conservation and application to problem solving pre and post collision Completely inelastic collisions Elastic collisions 2 28 11 Physics 231 Spring 2011 5 Momentum Definition p mv Note Vector magnitude mv same direction as velocity Components have sign px 0 for motion in positive x direction px 0 for motion in negative xdirection Definition is different from everyday use of word 2 28 11 Physics 231 Spring 2011 6 Impulse Momentum is changed by force applied over time here for x component px pxf pxi Fx t Fx t is also called impulse Jx Impulse is momentum change vector Impulse approximation often in collisions t is very small and F very large Other forces can then be neglected 2 28 11 Physics 231 Spring 2011 7 Clicker quiz Which ball generates a larger force on the block when colliding A Happy ball bounces back B Sad ball sticks to block C They generate the same force 2 28 11 Physics 231 Spring 2011 8 Momentum conservation Isolated system no force from the OUTSIDE acts on the system For isolated system p is constant This means px py and pz are constant Particularly useful for collisions solve problems by setting equal total momentum of the system before and after the collision 2 28 11 Physics 231 Spring 2011 9 Collisions Completely inelastic collision colliding objects stick together Momentum is conserved Kinetic energy not Elastic collision objects bounce back Momentum and kinetic energy are conserved Most collisions are not perfectly elastic some energy is converted into heat but often it s a good approximation for example a high quality bouncing ball 2 28 11 Physics 231 Spring 2011 10 Elastic collision Mass m1 moves with velocity vx1i and hits mass m2 which was at rest v1xf m1 m2 v1xi m1 m2 v 2xf 2m1 m1 m2 v1xi Where V1xf final velocity of m1 V2xf final velocity of m2 2 28 11 Physics 231 Spring 2011 11 Clicker Quiz What is the velocity of m1 after the collision if m1 m2 A vx1i B 0 C vx1i D vx1i 2 28 11 Physics 231 Spring 2011 12 Clicker Quiz What is the velocity of m2 after the collision if m1 m2 A vx1i B 0 C vx1i D vx1i E 2 vx1i 2 28 11 Physics 231 Spring 2011 13 Baron Muenchhausen 1720 1797 2 28 11 Physics 231 Spring 2011 14
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