Important Reminders
Last Lecture
Potential Energy diagrams, Potential Energy of springs
Exam #2 is tomorrow at 10 am.
Today
Covers material through last Thursday’ s lecture, last
Friday’s problem-solving and Pset, and Monday’s
MasteringPhysics assignment
Conclusion of energy discussion
The next important conservation law: Momentum
Important Concepts
Q&A review session tonight 7-8:30pm
Momentum is a vector.
MasteringPhysics due next Monday.
Think carefully about internal versus external forces.
No class next Friday.
Pset #7 due next Thursday.
8.01L Fall 2005
11/3/2005
8.01L Fall 2005
Work/Energy Summary
W = !E = EF " EI E = PE + KE KE = mv
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PEgravity = mgy PEspring = + 2 k ( L ! l0 )
! !
W = ! Fids
11/3/2005
Momentum
Very simple formula:
2
!
!
pTot = ! ( mi vi )
Note the vector addition!
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This quantity is conserved, another fundamental
property of our universe derivable from F=ma.
W = F ds cos(" )
Every force goes in the work term or in the PE
I could also claim F=ma is derivable from momentum.
Minima and maxima of the PE correspond to F=0,
which are equilibrium points. PE minima are stable
equilibrium points, maxima are unstable.
Sometimes it is impractical to include the whole
universe in the system under study.
In this case, we say momentum is conserved only if:
No net external forces acting on the system.
Or, study the system only over a very short time span.
8.01L Fall 2005
11/3/2005
8.01L Fall 2005
11/3/2005
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