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DYNAMICS - THEORY

    Harmonic Mass-Spring Motion


Horizontal Mass-Spring Motion

 

The simpliest type of vibrational motion is a mass moving back and forth horizontally due to a spring. To start the motion, the system is given either an initial displacement or velocity (or both). At any time, a force equilibrium and acceleration diagram can be constructed, similar to the diagram at the left.

Summing forces in the x-direction gives,

     -Fs = m ax

where Fs is the force in the spring, kx. This can be changed to,

     m d2x/dt2 + kx = 0

Because the basic differential equation describtion back and forth motion, it is refered to as harmonic motion.

     

Vertical Mass-Spring Motion
 

Similiarly, mass-spring motion in the vertical direction can also be modeled as a second order differential equation. Let the distance y represent the distance from the equilibrium position with gravity. The motion is started with an initial displacement and/or velocity. At any time, the forces can be summed, giving,

    FS - mg = -may

But the FS is the spring constant times the total displacement, FS = k(yS + yg). Also, noting yS = y, the equation becomes,

   k(y + yg) - mg = -may

However, ykg must equal 'mg' (see second graphic at left), and thus the 'mg' terms cancel, giving,

     m d2y/dt2 + ky = 0

which is the same as the horizontal equation. It is interesting to note, gravity does not effect the harmonic motion.

     

 

 


Spring-Mass System in Equilibrium

 

Using the vertical form of the differential equation, it can be written as

 
 

where the term ωn is called the natural frequency, and is given by

     

From differential equations, the general solution is

     y(t) = A sinωnt + B cosωnt

where the constants A and B must be determined from the initial conditions of position and velocity. This general solution can be verified by substituting back into the differential solution and noting all terms cancel.

Constant A is found by evaluating the initial condition dy(t)/dt at t = 0,

     

     

Displacement Cycle over Time
 

Constant B is found by evaluating y(t) at t = 0,

      B = y(0)

The diagram at the left shows a typical response for a spring-mass system. Notice that the response y(t) oscillates about the equilibrium position sinusoidally with a constant amplitude.

The frequency of oscillations, f, can be expressed in cycles per second (hertz) as

 
 

The time taken to complete one full oscillation is called the period τ,

 
 
     
    Equivalent Springs (Series and Parallel)

   

Many times, there will be more than one spring. But the vibration equations developed above (and in next several sections) are for only one spring. With the concept of equivalent springs, all spring systems can be grouped into parallel and series springs which will have equivalent spring constants.

     


Equivalent Spring K12 for
Two Springs in Parallel


Parallel Springs Stretched by Force F

 

Parallel Springs

If two springs are side-by-side, as shown in the diagram at the left, then the springs are in parallel. When parallel springs are stretched (or compressed) both springs will deflect the same amount, x. However, each spring can have a difference force, F1 and F2.

     F1 + F2 = F

     x k1 + x k2 = x k12

Canceling the deflection gives the equivalent relationship for two springs in parallel.

 
k1 + k2 = k12
 
     


Equivalent Spring K12 for
Two Springs in Series

 


Series Springs Stretched by Force F

 

Series Springs

Similar to two parallel springs, two springs in series can be reduced to a single equivalent spring constant, k12. When two springs in series are stretched (or compressed) the sum of the deflections in the two springs will equal the two deflection, x.

     x1 + x2 = x

However, the force in each spring (and the total force) will be the same.

     F/k1 + F/k2 = F/k12

Canceling the force F, gives,

 
1/k1 + 1/k2 = 1/k12
 
     
   
 
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