Download Sects. 4.9 & 4.10

Sect. 4.9: Rate of Change of a Vector
• Use the concept of an infinitesimal rotation to describe the
time dependence of rigid body motion.
• An arbitrary vector (or pseudovector) G (could, e.g., be
the position r), which is time dependent: G = G(t).
Time dependence of G depends on coord system in
which the observation is made. If body is rotating
&/or translating, G(t) is different when referred to body
axes (x, y, z) than when referred to space axes (x, y, z).
• In infinitesimal time dt, dG = (change in G) differs in
the 2 coord systems. Can write:
(dG)space = (dG)body + (dG)rot
where: (dG)rot  the effect of body rotation on G
• If G is fixed (constant) in the body, (dG)body = 0 so
(dG)space = (dG)rot
• In this case, G is rotating with the body & we can
use the results of the last section to write
(counterclockwise rotation of G):
(dG)rot  dΩ  G
where dΩ  differential vector angle associated with
the rotation (last section discussion)
 (dG)space = dΩ  G
• If G is not fixed in the body, (dG)body  0 so
(dG)space = (dG)body + dΩ  G
(1)
 In time dt, relation between rates of change of
G in 2 systems is obtained by dividing (1) by dt.
 (dG/dt)space = (dG/dt)body + ω  G
(2)
Have defined: Angular velocity ω dt  dΩ
Text makes a big deal about emphasizing that (dΩ/dt) = ω is
NOT a derivative of a function Ω(t), which the previous
section made a big point does not exist (for finite rotations).
Instead, (dΩ/dt) = ω = ratio of 2 infinitesimal quantities.
Direction of ω  instantaneous axis of rotation direction.
• More formal derivation of (2): See text p. 172-173. Uses
orthogonal transformation formalism.
(dG/dt)space = (dG/dt)body + ω  G (2)
• (2)  A GENERAL statement about transformation of
time derivatives of vectors between body & space
axes. Slight change of notation: space  s, body  r
(rotating system):
 (dG/dt)s = (dG/dt)r + ω  G
(2)
• Since G is arbitrary, take (2) as the definition of an
operator equation relating TIME DERIVATIVE
OPERATORS in the 2 systems:
(d/dt)s = (d/dt)r + ω 
(3)
• Will make repeated use of (3) (or (2))!

Or:
(dG/dt)s = (dG/dt)r + ω  G
(d/dt)s = (d/dt)r + ω 
(2)
(3)
– Note: This is, of course, for a body rotating without
translating!
• Convenient & useful to express angular velocity ω
in terms of Euler angles & their time derivatives.
– From Euler angle & infinitesimal rotation discussions:
General dΩ associated with vector ω consists of 3
successive infinitesimal rotations through Euler angles
,θ,ψ: Vector angular velocities associated with each:
ω  (d/dt) =  ωθ  (dθ/dt) = θ
ωψ  (dψ/dt) = ψ

ω = ω + ωθ + ωψ
ω = ω + ωθ + ωψ
(4)
• Practical difficulty with (4): Directions of ω,
ωθ, ωψ are not only not orthogonal, they are
not even symmetrical! Recall Euler angle
discussion for directions: ω along space z
axis, ωθ along line of nodes, ωψ along body z
axis.
• Can always get components of ω, ωθ, ωψ with
respect to any set of axes by use of individual
orthogonal transformations B, C, D & total
A = BCD of Euler angle discussion.
For example components of:
• ω (|| z axis) along the body axes:
(ω)x =  sinθ sinψ, (ω)y =  sinθ cosψ, (ω)z =  cosθ
• ωθ (|| ξ axis or line of nodes) along the body axes:
(ωθ)x = θ cosψ, (ωθ)y = - θ sinψ, (ωθ)z = 0
• ωψ (|| z axis) along the body axes:
(ωψ)x = 0,
(ωψ)y = 0,
(ωψ)z = ωψ = ψ
• Components of the total ω = ω + ωθ + ωψ along the body
axes (add component by component):
ωx =  sinθ sinψ + θ cosψ, ωy =  sinθ cosψ - θ sinψ
ωz =  cosθ + ψ
• Components of these along other axes? See problem 15.
• Time dependence of Cayley-Klein & Euler parameters? See
problem 16.!
Sect. 4.10: Coriolis Effect;
“Fictitious” Forces (from Marion)
• For transformation between time derivatives from space
system (s) and rotating system (r) we had:
(d/dt)s = (d/dt)r + ω 
(3)
• (3): Basic kinematical law for discussing dynamics of rigid
body motion (Ch. 5). Not restricted to rigid bodies! Valid
whenever we want to discuss the motion of a particle or
particles relative to a rotating coordinate system.
• Important application: Motion relative to coordinates of the
rotating Earth. “The Coriolis Effect” comes in. An example of
“fictitious” or non-inertial forces! Next, we will discuss noninertial forces in general. After the general discussion, then
application to Coriolis Effect. My treatment is similar to but much
more general than the text’s. It comes mainly from Marion’s text.
Non-Inertial Frames
General Discussion
• Inertial Reference Frame:
– Any frame in which Newton’s Laws are valid!
– Any reference frame moving with uniform (nonaccelerated) motion with respect to an “absolute” frame
“fixed” with respect to the stars.
• By definition, Newton’s Laws are only valid in
inertial frames!
F = ma: Not valid in a non-inertial frame!
• By definition, the concept of “Force”, as defined in Ch.
1 (& by Newton) , is only valid in inertial frames!
• It’s always possible to find an inertial frame to do the
dynamics of a system!
“Fictitious Forces”
• Some problems (like rigid body rotation!): Using an inertial
frame is difficult or complex. Sometimes its easier to use a
non-inertial frame!
• “Fictitious Forces”: If we are careful, we can the treat
dynamics of particles in non-inertial frames.
– Start in inertial frame, use Newton’s Laws, & make the
coordinate transformation to a non-inertial frame.
– Suppose, in doing this, we insist that our eqtns look like
Newton’s Laws (look like they are in an inertial frame).
 Coordinate transformation introduces terms on the “ma”
side of F = mainertial. If we want eqtns in the non-inertial frame
to look like Newton’s Laws, these terms are moved to the “F”
side & we get: “F” = manoninertial. where “F” = F + terms
from coord transformation  “Fictitious Forces” !
Non-Inertial Frames
• The most common example of a non-inertial frame =
Earth’s surface! Another is rigid body rotation.
• We usually assume Earth’s surface is inertial, when it is not!
– A coord system fixed on the Earth is accelerating (Earth’s
rotation + orbital motion) & is thus non-inertial!
– For many problems, this is not important. For some, we
cannot ignore it!
• Will do rotating coordinate systems. However, to understand
more about “fictitious forces” its simplest to first consider
only translational motion.
• Consider an inertial coord system S: Cartesian coordinate axes
(X,Y,Z). Coord system S: Cartesian coordinate axes
(X,Y,Z). S is in accelerated translational motion with
respect to S. Assume motion is such that axes (X,Y,Z)
remain parallel to (X,Y,Z). (No rotation for now!)
Translating Coordinate Systems
• Consider a point P in space, located at r = (x,y,z)
& r = (x,y,z) with respect to O & O.
O position = r0 = (x0,y0,z0) in S:
• Clearly:
r = r + r0
 x = x + x0
y = y + y0
z = z + z0
r = r + r0
 Velocity: v = (dr/dt) = (dr/dt) + (dr0/dt)
Or:
v = v + v0
 Acceleration: a = (dv/dt) = (dv/dt) + (dv0/dt)
a = (d2r/dt2) = (d2r/dt2) + (d2r0/dt2)
Or:
a = a + a0
• Suppose particle, mass m, is at P, acted on by
force F.
 Newton’s 2nd Law (only valid in inertial frame S):
ma = F
or
m(d2r/dt2) = F
• Newton’s 2nd Law (in S): ma = F or m(d2r/dt2) = F
• But, a = a + a0 
m(a + a0) = F
Or
m(d2r/dt2) + m(d2r0/dt2) = F
• To treat the dynamics of particle in the
accelerated frame S, rewrite as ma = F - ma0
Or
m(d2r/dt2) = F - m(d2r0/dt2)
(1)
(2)
• (1) & (2)  Mass times acceleration in accelerated frame =
Physical force (F) minus ma0 (or m(d2r0/dt2)). 2nd term
comes from coordinate transformation of Newton’s 2nd Law
from inertial frame S to non-inertial frame S.
 If we insist on writing ma = F
 We must have ma0 = a (fictitious) “force”, even
though its not a physical force!
• Newton’s 2nd Law (inertial frame S):
ma = F or m(d2r/dt2) = F
• Newton’s 2nd Law (accelerated frame S):
ma = F or
m(d2r/dt2) = F , Where
F = F - m(d2r0/dt2) = F - ma0
• ma0  A non-inertial or “fictitious force”: Comes
solely from kinematics of the coord transformation!
• Note: For non-accelerated (inertial) frames,
ma0 = 0

F = F
 Newton’s Laws are the same in all inertial
frames.