For a xed choice of system if there are no external forces acting on the system then the momentum of the system is constant The external force may be zero in one direction but not others The component of the system momentum is constant in the direction that the external force is zero The component of system momentum is not constant in a direction in which external force is not zero 1 Choose system 2 Identify initial and nal states 3 Identify any external forces in order to determine whether any component of the momentum of the system is constant or not Lecture 10 Momentum Law Conservation of Momentum System Consider a two particle system with no net external force acting on it In terms of masses and velocities we have that This is a vector equation a separate equation for each direction Strategy Momentum of a system a If there is a non zero total external force b If the total external force is zero then momentum is constant Recoil Relative speed The person jumps off of the cart with a speed u relative to the nal speed of the cart v The speed of the person v relation to the ground is v u v u is the speed of the ejected object relative to the thrower rocket Problem Solving Strategy Momentum Diagrams 1 Identify all objects in the system 2 Choose a reference frame 3 Draw a momentum diagram for a state of the system before interaction b state of the system after collision The diagram should include 1 Choice of unit vectors for direction of momentum 2 Show each mass element 3 Show the speed and arrow for direction of velocity of each mass element Rocket Motion in Empty space Fuel is continuously ejected from the rocket resulting in the rocket recoiling forward In a reference frame in which the rocket is at rest at time t the momentum diagrams for times t and t t are
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