THIS LECTURE
SIMPLE HARMONIC MOTION (SHM)
Motion near an equilibrium position can be approximated by SHM
Some examples of SHMs
An example of SHM: mass on spring k
x
kxxm From Newton’s and Hooke’s law we derivethe differential equation
)cos( tAxSolution:
mk /Angular frequency
Amplitude (A) and phase constant () are determined by the initial conditions
Period 2
T
Water molecules can vibrate in a number of ways with well-defined frequencies
O
H
Water molecules
Water molecules can vibrate in a number of ways with well-defined frequencies
O
H
Water molecules
An example of SHM: mass on springs
)cos( tAxSolution:
mkeff /Angular frequency Period 2
T
x
xkxkkxkxkxm eff )( 2121
From Newton’s and Hooke’s law we derivethe differential equation
21 kkkeff
xkxm eff
An example of SHM: mass on springs
)cos( tAxSolution:
mkeff /Angular frequency Period 2
T
x
xkxkxkxm eff 2211
From Newton’s and Hooke’s law we derivethe differential equation
21
21
kk
kkkeff
xxx 21
kxdx
dUF
constkxU
~
2
1~ 2
Near a position of stable equilibrium, U can be approximated by a parabola, i.e. an harmonic potential
x =0
U 2
2
1~ kxU
Motion near an equilibrium position can be approximated by SHM
SHM for a diatomic molecule
)1()(2)( 0rreDrU
Morse potential
r = distance between atoms
N2
The Morse potential, named after physicist P.M. Morse, is a convenient model for the potential energy of a diatomic molecule.
SHM for a diatomic molecule
m mr
)1()(2)( 0rreDrU
00
U
r
0
0
F
r
0r
0r
20 )()( rrDrU
For r~r0
00
U
r
)(2)( 0rrDrF
dr
dUrF )(
D