Physics 131 11/26/12

Prof. E. F. Redish 1

11/26/13 1 Physics 131

 Theme Music: Earth, Wind, & Fire Energy

 Cartoon: DaveCoverley Speed Bump

November 26, 2013 Physics 131 Prof. E. F. Redish

Lets look at electrical force moving an object

3 3

q m

q m

“Initial”: t1 Potential energy U

Where does the potential energy go when the charges move apart?

Electrical

11/26/13 Physics 131

Physics 131 11/26/12

Prof. E. F. Redish 2

Now lets make one object 1000 times heavier

M=1000m

4 4

q M q m

“Initial”: t1

How does the momentum of the two objects change? How does the KE of the two objects change?

Electrical

11/26/13 Physics 131

Momentum vs. energy   If we are changing the motion of two interacting

objects so the momentum of each change in the same way, it might be useful to look at the KE in terms of that momentum.

  Suppose each starts with 0 momentum and they move as a result of each other’s forces.

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!pA = !!pA = "!!pB = " !pB

KEA = 12 mAvA

2 = 12

mA2vA

2( )mA

= pA2

2mA

KEB = 12 mBvB

2 = 12

mB2vB

2( )mB

= pB2

2mB

Physics 131 11/26/12

Prof. E. F. Redish 3

Energy Conservation Total energy of object of interest is conserved unless external forces move the object of interest (i.e. do work on the object)

11/26/13 6 Physics 131

Child

Earth Gravitational

Kinetic Energy

All Interactions Crossing the Boundary do WORK

Energy Conservation for SYSTEM

11/26/13 7

Child

Earth Gravitational

Kinetic Energy

Total energy of system is conserved unless external forces move object(s) within the system (i.e. do work on the object(s))

Physics 131

Physics 131 11/26/12

Prof. E. F. Redish 4

11/26/13 10 Physics 131

Foothold ideas: Potential Energy

  For some forces work only depends on the change in position. Then the work done can be written U is called a potential energy.

  For gravity, Ugravity = mgh For a spring, Uspring = ½ kx2

For electric force, Uelectric = kCQ1Q2/r12

!F ! "!r = #"U

Foothold ideas: Conservation of Mechanical Energy

 Mechanical energy – The mechanical energy of a system of objects

is conserved if resistive forces can be ignored.

 Thermal energy – Resistive forces transform coherent energy

of motion (energy associated with a net momentum) into thermal energy (energy associated with internal chaotic motions and no net momentum)

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! KE + PE( ) = 0KEinitial + PEinitial = KEfinal + PEfinal