Driven Mass on a Spring, 3A60.39
Topic and Concept:
Oscillations, 3A60. Driven Mechanical Resonance
Location:
Cabinet: Waves and Sound (WS)
Bay: (A4 Right)
Shelf: #1
Abstract:
A function generator drives a spring with a suspended mass. The frequency is adjusted until resonance is reached.
Equipment 
Location 
ID Number 



Function Generator 


Pasco Driver 


Coaxial Cable 
Rod and tackle cabinet located near main lecture halls 

Springs 


Mass 


Rod and Clamps 
Rod and tackle cabinet located near main lecture halls 

Important Setup Notes:
 Demonstration may require practice.
Setup and Procedure:
 Vertically mount 3/4" rod to lecture bench using blue table clamp.
 Horizontally mount 1/4" rod to the vertical rod using a right angle clamp.
 Mount the Pasco driver to the 1/4" rod so that the driving component will move in a vertical direction.
 Place the function generator on the table near the rod, and plug it in.
 Connect the driver to the function generator with the two wires. Positive to positive. Ground to ground.
 Hang the spring from the driver.
 Hang the mass from the spring.
 When ready, turn on the function generator by pressing the "On" button.
 The mass will begin to oscillate in a vertical direction. Adjust the frequency of oscillation by changing the driving frequency set on the function generator so that the system approaches resonance (some practice beforehand is recommended).
Cautions, Warnings, or Safety Concerns:
 N/A
Discussion:
The dynamics of this system are described by a linear, inhomogeneous, second order, 2D (one spatial dimension, one time dimension) differential equation (see Hyperphysics Driven Oscillator). The inhomogeneous part comes from the driving force which is sinusoidal having a general form of
F(t) = F_{0} * cos(ω*t  φ_{d})
where F_{0} is the driving amplitude, ω is the driving frequency, and φ_{d} is the phase angle. F_{0} and ω are set on the function generator. The steadystate solution (letting t go to infinity) has the form
x(t) = A*cos(ω*t  φ) where A, and φ depend on the specifics of the driving force, the spring constant, damping, and the mass used. After more math, we find that A has the form
A = (F_{0}/m) / Sqrt((ω^{2}ω_{0}^{2})^{2} + 4*γ^{2}*ω^{2})
where m is the mass, γ encodes the damping information, and ω_{0} = Sqrt(k/m), k being the spring constant. Since the only variable in our expression for A is ω (we're only varying the driving frequency ω), A will be at a maximum when we minimize the denominator. This gives us ω_{resonance} = Sqrt(ω_{0}^{2}  2*γ^{2}). We call this value of ω the resonant frequency of the system.
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