Post on 13-Jan-2016
Sect. 4.10: Coriolis & Centrifugal Forces (Motion Relative to Earth, mainly from Marion)
• Summary: For motion in an accelerating frame (r), both
translating & rotating with respect to a fixed (f, inertial) frame: Velocities: vf = V + vr + ω r
Accelerations:
ar = Af + ar + ω r + ω (ω r) + 2(ω vr)
Newton’s 2nd Law (inertial frame):
F = maf = mAf + mar + m(ω r)
+ m[ω (ω r)] + 2m(ω vr)
“2nd Law” equation in the moving frame:
mar Feff F - mAf - m(ω r)
- m[ω (ω r)] - 2m(ω vr)
Motion Relative to Earth “2nd Law” in accelerating frame:
Feff mar F - mAf - m(ω r)
- m[ω (ω r)] - 2m(ω vr)
Transformation gave:
Feff F - (non-inertial terms)
• Interpretations:
- mAf : From translational acceleration of moving frame.
- m(ω r): From angular acceleration of moving frame.
- m[ω (ω r)]: “Centrifugal Force”. If ω r: Has magnitude mω2r. Outwardly directed from center of rotation.
- 2m(ω vr): “Coriolis Force”. From motion of particle in moving system (= 0 if vr = 0)
More discussion of last two now!
≈ 0 for motion near Earth
• Motion of Earth relative to inertial frame: Rotation on axis causes small effects! However, this dominates over other (much smaller!) effects:
ω = 7.292 10-5 s-1 ;
ω2Re = 3.38 cm/s2 = Centripetal acceleration at equator
2ωvr 1.5 10-4 v = max Coriolis acceleration
( 15 cm/s2 = 0.015g for v = 105 cm/s)
Even Smaller effects!– Revolution about Sun– Motion of Solar System in Galaxy– Motion of Galaxy in Universe
Also, ω = (dω/dt) ≈ 0
• Coordinate systems (figure): z direction = local vertical
– Fixed: (x,y,z) At Earth center– Moving: (x,y,z) On Earth surface
• Mass m at r in moving system.
• Physical forces in inertial system: F S + mg0
S Sum of non-gravitational forces
mg0 Gravitational force on m
g0 Gravitational field vector, vertical
(towards Earth center; along R in fig).
• From Newton’s Gravitation Law:
g0 = -[(GME)eR]/(R2)
G Gravitational constant, R Earth radius
ME Earth mass, eR Unit vector in R direction
– Assumes isotropic, spherical Earth– Neglects gravitational variations due to oblateness; non-
uniformity; ...
• Effective force on m, measured in moving system is thus: Feff S + mg0 - mAf - m(ω r)
- m[ω (ω r)] - 2m(ω vr)
• Earth’s angular velocity ω is in z direction in inertial system (North): ω ωez
ez unit vector along z
Earth rotation period T = 1 day
ω = (2π)/T = 7.3 10-5 rad/s
(Note: ω 365 ωes)
• ω constant ω 0 Neglect m(ω r)
• Consider mAf term in Feff & use again formalism of last time (rotation instead of translation):
Af = (ω Vf ) = [ω (ω R)]
• Effective force on m is:
Feff S+mg0 - (mω) [ω (r + R)] - 2m(ω vr)
• Rewrite as: Feff S + mg - 2m(ω vr)
Where, mg Effective Weight
g Effective gravitational field (= measured gravitational acceleration, g on Earth surface!)
g g0 - ω [ω (r + R)]
• Considering motion of mass m, at point r near Earth surface. R = |R| = Earth radius. |r| << |R|
ω [ω (r + R)] ω (ω R)
Effective g near Earth surface:
g g0 - ω (ω R)
• If m is at point r far from Earth surface, must consider both R & r terms. Effective g for any r:
g = g0 - ω [ω (r + R)]• Second term = Centrifugal force per unit mass
(Centrifugal acceleration).
• Centrifugal force: – Causes Earth oblateness (g0 neglects). Goldstein
discussion, p 176
– Earth Solid sphere. Earth Viscous fluid with solid crust.
– Rotation “fluid” deforms,
Requator - Rpole 21.4 km
gpole - gequator 0.052 m/s2
– Surface of calm ocean water is g instead of g0.
Deviation of g fromlocal vertical direction!
• Summary: Effective force:
Feff = S + mg - 2m(ω vr) (1)
Where, g = g0 - ω [ω (r + R)] (2)
Often,g g0 - ω (ω R) (3)
These are all we need for motion near the Earth!
Direction of g• Consider: g = g0 - (ω) [ω (r + R)] (2)
• Effective g = Eqtn (2). Consider experiments.
Magnitude of g: Determined by measuring the period of a
pendulum (small θ). DIRECTION of g: Determined by the direction of a “plumb bob” in equilibrium.
• Magnitude of 2nd term in (2):
ω2R 0.034 m/s2 (ω2R)/(g0) 0.35%
• Direction of 2nd term in (2): Outward from the axis of the rotating Earth. Direction of g = Direction of plumb bob = Direction of the vector sum in (2). Slightly different from the “true” vertical line to the Earth’s center. (Figure next page!)
• Direction of plumb bob = Direction of
g = g0 - (ω) [ω (r + R)] (2)
• Figure: (r in figure = r in previous figures!) Deviation of g from g0 direction is exaggerated!
r = R + z where z = altitude
Coriolis Effects• Effective force on m near Earth:
Feff = S + mg - 2m(ω vr)
- 2m(ω vr) = Coriolis force. Obviously, = 0 unless m moves in the rotating frame (moving with respect to Earth’s surface) with velocity vr.
• Figure again:
- 2m(ω vr) = Coriolis force.
• Northern Hemisphere: Earth’s
angular velocity ω is in z direction
in inertial system (North) ω ωez
ez unit vector along z (Figures):
In general, ω has components
along x, y, z axes of the rotating
system. All can have effects,
depending on the direction of vr.
• Most dominant is ω component which is locally vertical in rotating
system, that is ωz Component along local vertical.
- 2m(ω vr) = Coriolis force, Northern hemisphere.
– Consider ωz only for now.
• Particle moving in locally horizontal plane (at Earth surface): vr has no vertical component.
Coriolis force has horizontal component only, magnitude = 2mωzvr & direction to right of particle motion (figure).
Particle is deflected to right of the original direction:
• Magnitude of (locally) horizontal component ofCoriolis force ωz = (locally)
vertical component of ω (Local)
vertical component of ω depends on
latitude! Easily shown:
ωz = ω sin(λ), λ = latitude angle
(figure). ωz = 0, λ =0 (equator);
ωz = ω, λ = 90 (N. pole)
Horizontal component of Coriolis force, magnitude = 2m ωzvr
depends on latitude! 2mωzvr = 2mωvrsin(λ)• All of this the in N. hemisphere! S. Hemisphere: Vertical
component ωz is directed inward along the local vertical. Coriolis force & direction of deflections are opposite of N. hemisphere (left of the direction of velocity vr )
• Coriolis Deflections: Noticeable effects on: • Flowing water (whirlpools)
• Air masses Weather.
Air flows from high pressure
(HP) to low pressure (LP)
regions. Coriolis force deflects it. Produces
cyclonic motion. N. Hemisphere: Right
deflection: Air rotates with HP on right, LP
on left. HP prevents (weak) Coriolis force
from deflecting air further to right.
Counterclockwise air flow!
S. Hemisphere: Left deflection.
(Falkland Islands story)
Bathtub drains!
• More Coriolis Effects on the Weather:• Temperate regions: Airflow is not along pressure isobars due to the
Coriolis force (+ the centrifugal force due to rotating air mass).• Equatorial regions: Sun heating the Earth causes hot surface air to rise
(vr has a vertical component).
In Coriolis force need to account ALSO for (local) horizontal components of ω
Northern hemisphere: Results in cooler air moving
South towards equator, giving vr a horizontal
component . Then, horizontal component of Coriolis
force deflects South moving air to right (West) Trade
winds in N. hemisphere are Southwesterly.
Southern hemisphere: The opposite!
No trade winds at equator because Coriolis force = 0 there
All is idealization, of course, but qualitatively correct!