Damped Harmonic Motion with a Sinusoidal Driving Force

  1. 1. An oscillator with mass 0.5 kg, stiffness 100 N/m, and mechanical resistance 1.4 kg/s is driven by a sinusoidal force of amplitude 2 N. Plot the speed amplitude and the phase angle between the force and speed as a function of the driving frequency and find the frequencies for which the phase angle is 45°.

    2. m[tex]\ddot{x}[/tex] +Rm[tex]\dot{x}[/tex]+kx=Fosin[tex]\omega[/tex]t

    see attachment for rest of equations

    3. m= 0.5 kg, s=100N/m, Rm=1.4 kg/s, Fo=2N

    So my first question is this, is omega the independent variable in this case? Meaning, I solve everything that I am able to and leave omega alone. Also, is the differential equation in #2 the right form?

    I am confused at how I obtain the equation of motion so that I can plot this.

    Attached Files:

  2. jcsd
  3. vela

    vela 12,443
    Staff Emeritus
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    Yes, ω is the driving frequency, so it's the independent variable for what's being asked in the problem. I'm not sure what you mean by "leaving ω alone." You want to express the speed amplitude and the phase angle as a function of ω and the constant parameters of the system.

    Yes, your differential equation is correct. It is the equation of motion for the system. Note that k is the spring constant, which you called also called s in the other equations.

    I think you have a typo in your equation for the phase angle. Also, what does c represent in that formula?
  4. The equation for the phase angle is wrong it should be tan[tex]^{-1}[/tex](H)
    where H=[tex]\frac{\omega*m-k/\omega}{R_{m}}[/tex].

    I'm still a little confused about what they are asking for in regards to plotting.
  5. vela

    vela 12,443
    Staff Emeritus
    Science Advisor
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    By "speed amplitude," I assume the problem is asking for the amplitude of v(t). It will depend on ω.
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