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What to use when:
RIGID BODIES
When you are given the MI (through radius of gyration) and geometry of the body, you must use rigid
body mechanics, especially if the body is in plane motion or rotation.
1.
You will most certainly need to use
kinematics
in nearly all the problems (along with kinetics,
principles of work/energy, and principles of linear/angular momentum equations need to be
applied). In general, if you get more unknowns than the number of equations, you should
attempt using kinematics (v=r
ω
, a = r
α
, relative velocity and relative acceleration equations,
etc.).
2.
Use kinetics (
∑
F
x
,
∑
F
y
, and
∑
M
o
) if accelerations and angular accelerations
are involved. In such
cases, apply
∑
M
o
first, as it usually gives angular acceleration first. You may have to use
kinematics in the process (such as that for a
t
and a
n
). If you need any other forces, you will then
apply the force equations. Note that you will need to use the relative acceleration equation if
the body is moving (rotating, usually) with respect to another accelerating body. If there are any
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 Spring '09
 ENES

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