Driven un-damped harmonic motion

  • #1

Homework Statement



A particle of mass 'm' is subject to a spring force, '-kx', and also a driving force, Fd*cos(wdt). But there is no damping force. Find the particular solution for x(t) by guessing x(t) = A*cos(wdt) + B*sin(wdt). If you write this in the form C*cos(wdt - [tex]\phi[/tex]), where C > 0, what are C and [tex]\phi[/tex]? Be careful about the phase (there are two cases to consider).

Homework Equations



basic inhomogeneous second-order equation solving.

The Attempt at a Solution



So I got the equation F = Fd*cos(wdt) - k*x = m*x'', and put it in the form x'' + w2*x = F*cos(wdt), where w2[tex]\equiv[/tex] k/m, and F [tex]\equiv[/tex] Fd/m. I took the first and second derivative of the equation they told us to use for our guess, and subbed them into x'' + w2*x = F*cos(wdt). I ended up with A(w2 - wd2)*cos(wdt) + B(w2 - wd2)*sin(wdt) = F*cos(wdt). I therefore deduced that F = A(w2 - wd2). Therefore the particular solution would be xp(t) = A(w2 - wd2)*cos(wdt). Is there anyway that this can somehow simplify for the form they want us to put it in, C*cos(wdt - [tex]\phi[/tex]), or did I do something wrong?

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
ehild
Homework Helper
15,543
1,912
You know F and have to express A and B in terms of F, wd and w.

ehild
 
  • #3
ok, so then I get x(t) = [tex]\frac{F}{w^2-wd^2}[/tex]*cos(wd2*t) for the particular solution. I still can't figure out how this can take the form that is asked for.
 
  • #4
ehild
Homework Helper
15,543
1,912
ok, so then I get x(t) = [tex]\frac{F}{w^2-wd^2}[/tex]*cos(wd2*t) for the particular solution. I still can't figure out how this can take the form that is asked for.

It is [tex]x(t) = \frac{F}{w^2-wd^2}*\cos{(w_d*t)}[/tex]

instead and has the desired form if w>wd.
[tex]C = {\frac{F}{|w^2-wd^2|}, \Phi =0[/tex]

In the opposite case

[tex]x(t) =- \frac{F}{|w^2-wd^2|}*\cos{(w_d*t)}[/tex]

you have to include a phase constant of pi:

[tex]C = {\frac{F}{|w^2-wd^2|}, \Phi =\pi[/tex]

[tex]x(t) =\frac{F}{|w^2-wd^2|}*\cos{(w_d*t+\pi)}[/tex]

ehild
 

Related Threads on Driven un-damped harmonic motion

  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
2
Views
707
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
4
Views
374
  • Last Post
Replies
0
Views
4K
  • Last Post
Replies
3
Views
808
  • Last Post
Replies
4
Views
239
  • Last Post
Replies
5
Views
9K
  • Last Post
Replies
8
Views
1K
  • Last Post
Replies
11
Views
128
Top