may the force be with you! - sjtuweiwang/sites/www... · 2019. 5. 16. · weak force • weak force...
TRANSCRIPT
May the force be with you!
Fundamental vs. non-fundamental force
TosolveNewton’smechanicsproblems,oneneedtoknowforces.Therearefourfundamentalforces:Thegravita=onalforcelawfoundbyNewtonisfundamental.Bynow,allfundamentallawsofotherthreeforcesareknownaswell:electromagne=c,weakandstrong.
Friction
The E&M force: Coulomb’s law
Strong force
• Strongforceistheonethatbindstogethertheneutronsandprotonsinnuclei.• Thisfundamentalforcewasfinallyunderstoodin1970swhencolorchargeandatheoryofcolorinteractionsareproposedandverified.(Chromodynamics)• However,solvingthe“NewtonEquation”isveryverydifficulty.Theonlywaytodothisisusingalargescalecomputer!
Weak force
• Weakforceistheonethatchangestheprotonintoaneutronandhenceamongdifferentelements.• Weakforceisthefirststepinthesunburning.• Thefundamentaltheoryofweakforcewasproposedin1967,wasverifiedovertheyears.• Theweakforcehasthesameoriginastheelectromagneticforce.• Theweakforcealsogivethemasstotheelectron!
Pseudo force due to non-inertial frame• Ininertialframes,alltheforcesare“physical”.Alllawsininertialframesarethesame.• However,whenworkinginnon-inertialframe,thereareextraforcesinNewton’sequation.Forexamplewhenaframeisacceleration,oneneedstotakeintoaccountthisspurious“force”• Sometimeunavoidable.LiketheCoriolisforceontheEarth->Foucaultpendulum.
Question
Question
Braess' paradox
Coordinates transformation
Coordinate translation
Translation
Translational invariance
• Thenewton’slawofphysicsisvariantundertranslation
• Theforcedoesnotchangeundertranslation
• Theaccelerationdoesnotchangeundertranslation
• Thisistheoriginofmomentumconservation
• Webelievealllawsofphysicsareinvariantunder
translation
Time translational symmetry
•
Rotations
11 12 13
21 22 23
31 32 33
'''
x a a a xy a a a yz a a a z
⎛ ⎞ ⎛ ⎞⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟=⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠
Rotational matrix
•
Rotational symmetry
•
Galileo transformation
• Twocoordinatesarerelatedbyconstantspeed,sowehavetwocoordinates
ThisiscalledGalileantransformation.
0
0
0
'''
'
x
y
z
t tx x v ty y v t
z z v t
=
= −
= −
= −
0
''t tr r v t=
= −! ! !
Newton’s equations are invariant under Galileo transformation
• Galileanprincipleofrelativity.Nottruewhenvelocityislarge
0
''t tr r v t=
= −! ! ! 0 0dv
dt=
!'a a=! !
'F F=! !
F ma=! ! ' 'F ma=
! !