Lecture 9 The Coriolis Force Life in an Accelerating Reference Frame Suppose we step into a subway car with a rest Teflon floor When the train starts to accelerate we slide backward until we hit the back wall From our perspective in the accelerating car there is a backward time apparent force that rammed us into the back wall Our actual position Our original position with respect to car Our motion with respect to car On the rotating earth we feel similar apparent forces because it takes a force to accelerate an object in a circle Motion of rotating object Centripetal force needed to keep object going in circle Apparent centrifugal force felt by rotating observer These apparent forces are Centrifugal force Coriolis force Centrifugal Force Apparent force pulling Gravity outward from Earth s Centrifugal polar axis Effective Slightly changes the gravity apparent gravity 0 5 weaker at the equator than at poles Sea level and Earth surface are pulled 20 km 0 3 further from the center of the earth at the equator than at poles By redefining gravity to point perpendicular to sea level everywhere we can absorb centrifugal force into gravity Coriolis force or effect An apparent force on moving objects on the rotating Earth Gaspard Coriolis 1848 EOM 6 12 Coriolis force increases proportional to Rotation rate of the planet Latitude Speed of the object It accelerates moving objects to their right in the Northern Hemisphere and to their left in the Southern Hemisphere There is no Coriolis force at the equator It is at right angles to the motion changing a moving object s direction but not its speed Seattle A commercial jet flying from Seattle to Denver feels a sideways Coriolis force of 0 1 of gravity If the jet starts flying toward Denver it must imperceptibly tip its wings to compensate else it would be deflected to Salt Lake City Denver Coriolis Force SLC Coriolis Force and Air Motion EOM 6 14 Isobar contour of contant pressure at a given height Closely packed contours Strong PGF Strong wind EOM Except near surface winds are nearly geostrophic blowing along the isobars at a speed for which the Coriolis force CF nearly balances the PGF Effect of Friction EOM 6 18 Friction causes near surface wind to spiral out of high in toward low pressure Less friction over water than land so surface wind blows more along isobars
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