Difference between revisions of "Forest UCM Osc Damped"

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==Energy ==
 
==Energy ==
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A damped oscillator is a system that is loosing energy as a function of time ( energy is leaving the system usually in the form of heat).  The system starts out with a total energy at time <math>t=0</math> of <math>E(0)</math>.  A frictional force exists causing the system to do work (W_f) and thereby lose energy.
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:E(t) = E(0) + W_f
  
  
 
[[Forest_UCM_Osc#Damped_Oscillations]]
 
[[Forest_UCM_Osc#Damped_Oscillations]]

Revision as of 12:27, 6 October 2014

1-D Damped Oscillations

An example of a damped oscillator is the case of a mass attached to a spring with one end fixed. The block slide on top of a table and feels a frictional force from the table.


Newton's 2nd Law

As in the case of air resistance, it was assume that there was a frictional force proportional to the velocity of the oscillation body. The same assumption is made for a block oscillating via a spring with friction between the block and the table top.


\sum \vec{F}_{ext} = -k\vec r - b \vec \dot v = m \vec \ddot r
Fext=kxb˙x=m¨x: in 1-D

or

m¨x+kx+b˙x=0

or

¨x+kmx+bm˙x=0


let

km=ω20= undamped oscillation frequency
bm2β= damping constant

then

¨x+2β˙x+ω20x=0

Solve for the Equation of Motion

As see in section Forest_UCM_Osc_SHM#Equation_of_motion, you can determine solutions to the above by writing the analogous auxilary equation:

(O2+2βO+ω20)x=0Oddt

Setting the term in parentheses to zero and using the quadratic formula

O=2β±(2β)24ω202=β±β2ω20
(O+β+β2ω20)(O+ββ2ω20)x=0


You have change the second order differential equation into two first order differential equations

(ddt+β+β2ω20)x=0
dxx=(ββ2ω20)dt
x=e(ββ2ω20)t


(ddt+ββ2ω20)x=0
dxx=(β+β2ω20)dt
x=e(β+β2ω20)t

constructing a complete solution from the two solutions (orthogonal functions) above.

x=(C1eβ2ω20t+C2eβ2ω20t)eβt

Undamped oscillator

If β = 0

Then

x=(C1eβ2ω20t+C2eβ2ω20t)eβt
=(C1eiω0t+C2eiω0t) the SHM solution derived before at Forest_UCM_Osc_SHM#Equation_of_motion

Under damped Oscillator

β<ω0

In this case the term

β2ω20=(1)(ω20β2=iω20β2iω1



x=(C1eβ2ω20t+C2eβ2ω20t)eβt
=(C1eiω1t+C2eiω1t)eβt
=Aeβtcos(ω1tδ)


There are two terms above, the first term is an exponential decay and the second is the usual harmonic oscilator.

They combine to produce oscillations whoase amplitudes decay with time.

Over damped Oscillator

β>ω0


x=(C1eβ2ω20t+C2eβ2ω20t)eβt
=(C1e(β+β2ω20)t+C2e(ββ2ω20)t)


For the overdamped case you have two exponentials with negative exponents

β+β2ω20<0

Critically damped Oscillator

β=ω0


x=(C1eβ2ω20t+C2eβ2ω20t)eβt
=Aeβt

In this case the method of using the auxilary equation has yielded two identical solutions. A second solution should exist.

Problem 5.21 and 5.24

x=C1eβt+C1teβt

Physics Example; The LRC circuit

An electrical circuit composed of an inductor (L) , a resister (R), and a capacitor (C) is known as an LRC circuit.

The resistor in the circuit serves to simply resists the movement of charge through the circuit ( analogous to friction; causing energy to leave the system in teh form of heat) such that the voltage drop accross the resistor is given by

V=R˙q

The capacitor in the ciruit plays role of storing charge ( like a spring stores energy) such that it become harder to store charge on the capacitor as you increase the amount of charge you try to store in it. The voltage drop across a capacitor is given by

V=Cq

An Inductor trys to resist the change in the rate that the current moves around the system (like inertia; produce by inducing a reverse EMF). The voltage drop across an inductor is given by

V=L¨q

If you apply conservation of energy (potential) by traversing the circuit in a complete loop (Kirchoff's 2nd law) then you can write the conservation law as

L¨q+R˙q+1Cq=0

The above differential equation has the same form as the damped oscillator differential equation where

2β=RL and ω20=1LC

Energy

A damped oscillator is a system that is loosing energy as a function of time ( energy is leaving the system usually in the form of heat). The system starts out with a total energy at time t=0 of E(0). A frictional force exists causing the system to do work (W_f) and thereby lose energy.

E(t) = E(0) + W_f


Forest_UCM_Osc#Damped_Oscillations