1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Linear Damping Model

  1. Jun 25, 2008 #1
    Hi All

    This is my first post and I was hoping that someone could help. I have the following attached question regarding a linear damping model (in this case a seismograph).

    (1) Draw a force diagram.

    If W=weight, R= air resistance, and H= spring force, then I get W pointing down and both R and H pointing up. Is this right? Should there be another force as well (i.e. at the pointer and in the damper)

    (2) Express forces on particle in terms of given variables and parameters and unit vector i.

    I'm confused on this one. Can someone help?

    (3) Express displacement vector p of mass relative to origin in terms of x, y, d and i. Write down equation of motion of particle, and hence show that x(t) satisfies the differential equation md^2x/dt^2 +rdx/dt+kx=mg+kl+md^2y/dt^2


    For (3) I get the following (can someone check):

    We are applying Newton's second law here, but there is one extra feature: the acceleration of the box comes in as relative motion. So whatever expression we get for the net force, we need to add a m d^2y / dt^2 to it. I will simply put that into the equation "by hand."

    I have a coordinate system with the positive direction upward. So in the Free Body diagram of the mass there is a weight (w) acting downward, a spring force (f) acting upward, and a resistive force (R) from the piston acting downward.

    f = kl = k(x - l_0)

    w = mg

    R = r \frac{dx}{dt}

    So Newton's second in the vertical direction gives:
    m \frac{d^2x}{dt^2} = -mg + k(x - l_0) - r \frac{dx}{dt} + m \frac{d^2y}{dt^2}

    (4) Suppose displacement vector y is given by y=acos(ft), where a is input amplitude, and f is omega (forcing angular frequency). Show that M, the given ratio of the amplitude A of the steady state output oscillations to a is given by:

    M=A/a=mf^2/SQRT((k-mf^2)^2+r^2f^2)

    I havent got a clue with this one. Any help would be greatly appreciated

    Many thanks
     

    Attached Files:

  2. jcsd
  3. Jun 26, 2008 #2
    For part (2), am I right in thinking that the following are the forces acting on the particle:

    W=-mgi
    R=rxi
    H=k(l-l0)s


    note the x term in R has a dot on the top (ie function of velocity I think
     
  4. Jun 26, 2008 #3
    This block really sucks.

    The dot means the its the derivative of x(t)...

    I think somebody might be more inclined to help if you didnt present the problems in Latex code.

    Can somebody help? Please?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Linear Damping Model
  1. Damped Oscillator (Replies: 6)

  2. Damped Oscillator (Replies: 2)

Loading...