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# Week4b - Hypothetical You applied for a summer job in...

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Hypothetical You applied for a summer job in Scotland - Carey gives reference Scotland (Lockerbie) Pan Am Flight 103 first project. Know exact location and velocity of plane at time of explosion. Etc etc. Special Case: Where Net External Forces are Zero In the special case where there is no net external force acting on the system, we have so that M v cm = P = " m i v i = constant. __________________________________________________ Ma cm = dP dt = d(M v cm ) dt = 0, This result is known as: the law of conservation of momentum. i.e MOMENTUM BEFORE = MOMENTUM AFTER MOMENTUM BEFORE = MOMENTUM AFTER

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This law is very useful for solving the motion of colliding particles, whereas the law of the conservation of energy cannot usually be used because of the non-conservative nature of the internal forces acting between particles in the interaction which can change the total mechanical energy. __________________________________________________________ Examples of motion with zero total external force. 1. Explosions M V m v Initial momentum P = 0 Final momentum P = 0 = " m i v i i.e. 0 = -MV + mv Remember in a closed system where there are no external forces acting - momentum is conserved. i.e. the total amount of momentum is constant. This is true whether energy is conserved or not. Collisions When bodies come together and interact strongly for a short time. F A on B A B F B on A Newtons 3rd. Law: F BA = - F AB And in a closed system of 2 bodies there are no external forces.
Collisions may be elastic or inelastic. Collisions may be elastic or inelastic. Note: All collisions of objects larger than atomic scale are inelastic.

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