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Chapter 7: Linear Momentum

Chapter 7: Linear Momentum. Momentum: a measure of motion Force: a cause of change in motion What changes when a force is applied?. Linear Momentum : p = m v (vector!!!!!) the tendency of an object to pursue straight line motion Kinetic Energy in terms of momentum:.

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Chapter 7: Linear Momentum

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  1. Chapter 7: Linear Momentum • Momentum: a measure of motion • Force: a cause of change in motion • What changes when a force is applied? • Linear Momentum: p = mv(vector!!!!!) • the tendency of an object to pursue straight line motion • Kinetic Energy in terms of momentum:

  2. Impulse: the change in motion Example: A baseball with mass 0.14 kg and an initial speed of 30 m/s his hit with a bat and then rebounds in the opposite direction at a speed of 40 m/s. The bat and ball are in contact for 0.0020 s. Determine the size of the impulse on the ball and the magnitude of the average force of the bat on the ball.

  3. Example: A 51 kg teenager jumps to the ground from a chair 0.34 m high. She bends her knees slightly on landing, lowering herself by only 8.0 cm during her landing. What is the average force with which her feet hit the ground?

  4. Conservation of momentum • two bodies + action/reaction + no other forces • FAB = - FBA • => equal but opposite impulses • => DpA + DpB = 0 • When the net external force on a system is zero, the total momentum of that system is constant. • p1 + p2 + p3 + ... is constant • Collisions: m1v1 + m2v2 =m1v’1 + m1v’2

  5. Example: A Buick Park Avenue (m=1660 kg) with an initial speed of 8.0 km/hr collides head on with a Geo Metro (m=830 kg). As a result of the collision, the cars become entangled and so “sick together”. - What is the speed of the wreckage just after the collision? - How do the accelerations of each car compare? (look at the changes in their velocities) Example: A 60 kg ice skater initially at rest throws a 2 kg block of ice horizontally with a speed of 12 m/s. What is his recoil velocity? Ballistic Pendulum demo

  6. Collisions • Elastic Collisions • conserve KE (total KE is same before and after collision) • Inelastic Collisions • some KE is lost during collision (heat, sound, etc.) • Completely Inelastic Collisions • objects stick together • maximum possible loss of KE • In all collisions, the total momentum is conserved!

  7. Collisions: m1v1 + m2v2 =m1v’1 + m1v’2 • in 2 or 3 dimensions: take components! • m1vx1 + m2vx2 =m1v’x1 + m1v’x2 • m1vy1 + m2vy2 =m1v’y1 + m1v’y2 • Example: Two cars approach an intersection at right angles. After the crash they stick together. If one car has a mass of 1450 kg and is traveling north at 11.5 m/s and the other has a mass pf 1750 kg and is traveling east at 15.5 m/s, determine the speed and direction of motion of the wreckage just after the collision.

  8. Example: A billiard ball moving at 10 m/s along the positive x axis collides with an identical billiard ball at rest. After the collision, the incoming ball moves at a speed of 7.7 m/s at an angle 40º from the x axis. What is the speed and direction of the struck ball?

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