- #1
changyongjun
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Homework Statement
In general, I know that the dynamic equation of pendulum is
theta'' + (k/m)*thata' + (g/l)*sin(theta) = 0
, where k=friction co, m=mass, l=length of string, g=gravity.
But if the pendulum is placed in a constant draft, the equation has to be changed.
Assuming that the pendulum is suspended in a constant horizontal wind,
imparting a constant force w on the bob, the equation is
theta'' + (k/m)*thata' + (g/l)*sin(theta) - (w/m)*cos(theta)= 0
I cannot understand the term of (w/m)*cos(theta). If I rewrite the eq,
m*l*theta'' + (k*l*theta') + (m*g*sin(theta)) - (w*l*cos(theta)) = 0
( inertia )------(friction)--------( gravity )---------( what ?? )
My question is why the length of string, ' l ' affects the external force term.
In my oppinion, external force term is just ( w * cos(theta) ), but the answer is not :(.
Please somebody help me ...