University:
California Institute of Technology
Course:
EE/APh 40 | Physics of Electrical Engineering
Academic year:

2022
Views:

419

Pages:

10
Author:

StellarStrategist

8.4 Conservation of Momentum The law of conservation of momentum states that, in the absence of an external force, the momentum of a system remains unchanged. 8.4 Conservation of Momentum The force or impulse that changes momentum must be exerted on the object by something outside the object. • Molecular forces within a basketball have no effect on the momentum of the basketball. • A push against the dashboard from inside does not affect the momentum of a car. These are internal forces. They come in balanced pairs that cancel within the object. 8.4 Conservation of Momentum The momentum before firing is zero. After firing, the net momentum is still zero because the momentum of the cannon is equal and opposite to the momentum of the cannonball. 8.4 Conservation of Momentum The force on the cannonball inside the cannon barrel is equal and opposite to the force causing the cannon to recoil. The action and reaction forces are internal to the system so they don’t change the momentum of the cannon-cannonball system. • Before the firing, the momentum is zero. • After the firing, the net momentum is still zero. • Net momentum is neither gained nor lost. 8.4 Conservation of Momentum Momentum has both direction and magnitude. It is a vector quantity. • The cannonball gains momentum and the recoiling cannon gains momentum in the opposite direction. • The cannon-cannonball system gains none. • The momenta of the cannonball and the cannon are equal in magnitude and opposite in direction. • No net force acts on the system so there is no net impulse on the system and there is no net change in the momentum. 8.4 Conservation of Momentum In every case, the momentum of a system cannot change unless it is acted on by external forces. When any quantity in physics does not change, we say it is conserved. 8.4 Conservation of Momentum The law of conservation of momentum describes the momentum of a system: If a system undergoes changes wherein all forces are internal, the net momentum of the system before and after the event is the same. Examples are: • atomic nuclei undergoing radioactive decay, • cars colliding, and • stars exploding. 8.4 Conservation of Momentum think! Newton’s second law states that if no net force is exerted on a system, no acceleration occurs. Does it follow that no change in momentum occurs? 8.4 Conservation of Momentum think! Newton’s second law states that if no net force is exerted on a system, no acceleration occurs. Does it follow that no change in momentum occurs? Answer: Yes, because no acceleration means that no change occurs in velocity or in momentum (mass × velocity). Another line of reasoning is simply that no net force means there is no net impulse and thus no change in momentum. 8.4 Conservation of Momentum What does the law of conservation of momentum state?

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