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Dynamics cheat sheet

my dynamics notes - 12000.org

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Therefore the first shape vector is<br />

⎧ ⎫ ⎧<br />

⎨ϕ 11 ⎬ ⎪⎨<br />

ϕ 1 =<br />

⎩<br />

ϕ<br />

⎭ = 1<br />

⎪<br />

21<br />

⎩<br />

− ( k 11 −ω1 2m )<br />

( 11<br />

k12 −ω1 2m 12)<br />

⎫<br />

⎪⎬<br />

⎪⎭<br />

Similarly the second shape function is obtained. For ω 2 , let ϕ 12 = 1 and solve for<br />

⎡ ⎤ ⎡ ⎤ ⎧ ⎫ ⎧ ⎫<br />

⎣ k 11 k 12<br />

⎦ − ω2<br />

2 ⎣ m 11 m 12<br />

⎨ 1 ⎬ ⎨<br />

⎦<br />

k 21 k 22 m 21 m<br />

⎩<br />

22 ϕ<br />

⎭ = 0⎬<br />

⎩<br />

22 0<br />

⎭<br />

⎡<br />

⎤ ⎧ ⎫ ⎧ ⎫<br />

⎣ k 11 − ω2 2m 11 k 12 − ω2 2m ⎨<br />

12 1 ⎬ ⎨<br />

⎦<br />

k 21 − ω2 2m 21 k 22 − ω2 2m ⎩<br />

22 ϕ<br />

⎭ = 0⎬<br />

⎩<br />

22 0<br />

⎭<br />

Which gives one equation now to solve for ϕ 22 (the first row equation is only used)<br />

(<br />

k11 − ω 2 2m 11<br />

)<br />

+ ϕ22<br />

(<br />

k12 − ω 2 2m 12<br />

)<br />

= 0<br />

Hence<br />

ϕ 22 = − ( k 11 − ω2 2m )<br />

11<br />

(<br />

k12 − ω2 2m 12)<br />

Therefore the second shape vector is<br />

⎧ ⎫ ⎧<br />

⎨ϕ 12 ⎬ ⎪⎨<br />

ϕ 2 =<br />

⎩<br />

ϕ<br />

⎭ = 1<br />

⎪<br />

22<br />

⎩<br />

− ( k 11 −ω2 2m )<br />

( 11<br />

k12 −ω2 2m 12)<br />

Now that the two non-normalized shape vectors are found, the next step is to perform mass normalization<br />

1.4 Step 4. Mass normalization of the shape vectors (or the eigenvectors)<br />

Let<br />

µ 1 = ϕ T 1 [M] ϕ 1<br />

This results in a scalar value µ 1 , which is later used to normalize ϕ 1 . Similarly<br />

µ 2 = ϕ T 2 [M] ϕ 2<br />

⎫<br />

⎪⎬<br />

⎪⎭<br />

For example, to find µ 1<br />

⎧ ⎫T ⎡ ⎤ ⎧ ⎫<br />

⎨ϕ 11 ⎬<br />

µ 1 = ⎣ m 11 m 12<br />

⎨ϕ ⎦ 11 ⎬<br />

⎩<br />

ϕ<br />

⎭<br />

21 m 21 m<br />

⎩<br />

22 ϕ<br />

⎭<br />

21<br />

{ } ⎡ ⎤ ⎧ ⎫<br />

= ϕ 11 ϕ ⎣ m 11 m 12<br />

⎨ϕ ⎦ 11 ⎬<br />

21<br />

m<br />

⎩<br />

22 ϕ<br />

⎭<br />

21<br />

m 21<br />

}<br />

=<br />

{ϕ ⎧ ⎫<br />

⎨ϕ 11 ⎬<br />

11 m 11 + ϕ 21 m 21 ϕ 11 m 12 + ϕ 21 m 22<br />

⎩<br />

ϕ<br />

⎭<br />

21<br />

= ϕ 11 (ϕ 11 m 11 + ϕ 21 m 21 ) + ϕ 21 (ϕ 11 m 12 + ϕ 21 m 22 )<br />

6

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