Physics 7:1-4 Conservation of Momentum and Energy
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 7: Sections 1-4
- Section 7.1: Momentum is defined as the vector p = mv. Since v is a vector and is only multiplied the the scalar m, the result (momentum) must also be a vector. Force = ma = mΔv/Δt, which we can generalize to F = Δp/Δt. Note: Einstein realized that this formulation of force as "impulse" included the possibility that mass could change...and indeed it does when it is traveling at relativistic speeds.
- Section 7.2: Like energy, momentum in a system is conserved. The center of mass of the system, taken as a whole, cannot change unless acted on by an outside force. If individual parts of the system change their momentum (by changing direction or speed or mass), then the other parts of the system must change to compensate for this. This gives us a new tool for analyzing systems that include colliding particles that change speed and direction.
- Section 7.3: We can now define impulse by rewriting the momentum equation as impulse = FΔt = Δp. A force F applied to a body for a time Δt will change the momentum by Δp. The greater the force, or the longer the time, the greater the change in the momentum of the system affected by the force.
- Section 7.4: We now have two rules for changes to bodies, including bodies colliding with each other. Energy and momentum are both individually conserved, which gives us two independent rules for the system. Even though they involve m and v, each "law" can be applied by itself when we analyze a system.
- Definiion of force in terms of momentum:
- Change in momentume = Impulse
- Conservation of momentum in elastic collision
Read the following weblecture before chat: Momentum
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Change the masses of the balls to determine how mass changes momentum in collisions. Use the vector displays to determine the direction and relative sizes of velocity, momentum, and KE.
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