ATOC 184 Lecture12_Feb15_2012

ATOC 184 Lecture12_Feb15_2012 - Why and how the air moves...

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Unformatted text preview: Why and how the air moves What is a force? (Force = mass times acceleration) A force is something that causes a mass to accelerate. A common example is that of gravity. An object's weight can change if it goes into space or to another planet. This is because gravity may be weaker or stronger there than it is on the Earth. There are two kinds of forces A fundamental force is one that results from an interaction of particles and is independent of the frame of reference. Gravity from the previous slide is a fundamental force. An apparent force acts on all masses in a non-inertial frame of reference, such as a rotating reference frame. The force F does not arise from any physical interaction but rather from the acceleration a of the non- inertial reference frame itself. Fundamental Force Apparent Force What forces act on air? Four forces control the air motions: Pressure gradient force (fundamental) Gravitational force (fundamental) Frictional force (fundamental) Coriolis force (apparent or fictitous-results from the Earth’s rotation) The horizontal pressure gradient force An animation of the horizontal pressure gradient force: The Coriolis Force The Coriolis Force is an apparent force associated with the Earth’s rotation Consider the video at http://ca.youtube.com/watch?v=mcPs_OdQOYU The Coriolis Force: http://www.physics.oregonstate.edu/~mcintyre/co http://www.classzone.com/books/earth_science/terc/content/visua Another illustration of how the Coriolis Force works: Consider the concept of centripetal and centrifugal force. The centripetal force is the external force required to make a body follow a curved path. Another illustration of how the Coriolis Force works: The thick arrows show the directions that eastbound and westbound projectiles would seem to go as a result of the Coriolis effect in the absence of gravity. The eastbound projectile (red, upper horizontal arrow) would seem to drift away from the axis, while the westbound projectile (green, lower horizontal arrow) would seem to drift towards the axis Summary of the Coriolis Force: Causes objects to deviate to the right in the N. Hemisphere, and to the left in the Southern Hemisphere Affects the direction an object will move across the Earth’s surface, but has no effect on its speed Strongest for faster-moving objects, and nill for stationary objects Zero at the Equator, and strongest at the Poles Table 6.1 Newton’s second law for horizontal and vertical motions (PGF denotes the pressure gradient force) Type of Motion Newton’s Second law Horizontal motions above the boundary layer Acceleration = Horizontal PGF + Coriolis Force Horizontal motions within the boundary layer Acceleration = Horizontal PGF + Coriolis Force + Friction Vertical motions Acceleration = Vertical PGF + Gravity Force balances Hydrostatic balance: The gravitational force being balanced by the vertical pressure gradient force Force balances (Continued)...
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This note was uploaded on 03/12/2012 for the course ATOC 184 taught by Professor Gyakum during the Winter '12 term at McGill.

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ATOC 184 Lecture12_Feb15_2012 - Why and how the air moves...

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