Do Rocks Always Roll Downhill? Gedzelman, p 247 Upper-Level Winds PGF
Newton’s Laws of Motion Newton’s 1st Law An object at rest will remain at rest and an object in motion will remain at a constant velocity (same speed and same direction) if the net force exerted on it is zero An external force is required to speed up, slow down, or change the direction of air
Newton’s Laws of Motion Newton’s 2nd Law The net force exerted on an object equals its mass times its acceleration Sum of All Forces = Mass Acceleration Acceleration = Velocity Change / Time Acceleration = Change in Speed and/or Direction
Velocity, Acceleration and Force are Vectors Size and Direction Original Velocity New Velocity Original Velocity New Velocity Acceleration and Force Original Velocity New Velocity Original Velocity New Velocity Acceleration and Force
Uniform, Circular Motion Requires Acceleration Original Velocity New Velocity Acceleration directed toward center of circle Centripetal Original Velocity New Velocity Circular Path
Accelerated Frame of Reference You are glued to car’s floor and drop an egg. What happens if the car begins to accelerate? Inside the car, it looks a mystery force is attracting the egg to the back of the car. Your frame of reference is accelerating. Someone outside the car sees that the egg is just accelerating to the floor, you are accelerating with the box car. A force is accelerating the car. Their frame of reference is not accelerating. Splat! (rest ) time
Life on a Rotating Platform From perspective of person not on merry-go- round, path of ball is straight. From perspective of person on merry-go- round, path of ball deflects to left. There is an apparent force. (left click picture for animation) World Weather Project 2010 Courtesy of M. Ramamurthy U of Illinois, Urbana-Champaign Merry Go Round Link
Earth’s Rotation If viewed from space, earth is like a carousel! Northern Hemisphere rotates counterclockwise Southern Hemisphere rotates clockwise Gedzelman, p 240
Ball Appears to Deflect to the Right of the Observer Gedzelman, p 242 Deflection increases if: Rotation rate increases Speed of ball increases
Deflection Depends on Orientation of Axis of Rotation and Velocity Gedzelman, p 242 velocity Apparent Deflection No Deflection
Coriolis Force Varies with Latitude Gedzelman, p 243
Geostrophic Adjustment A.Parcel at rest initially accelerates toward lower pressure. B.Coriolis Force rotates parcel to right in NH. C.As parcel speeds up, Coriolis Force increases. D.Eventually (about a day), PGF equals CF and flow is parallel to isobars. World Weather Project 2010 Courtesy of M. Ramamurthy U of Illinois, Urbana-Champaign Animate Picture
Geostrophic Balance Pressure Gradient Force Coriolis Force Geostrophic Wind 5640 m 5700 m Geostrophic Wind Arises from a Balance Between the PGF and the Coriolis Force. PGF + Coriolis Force = 0 Technically, can exist only for East-West flow and for straight contours.
Geostrophic Balance Pressure Gradient Force Coriolis Force Geostrophic Wind 5640 m 5700 m The Balance Leads to the Wind Blowing Parallel to the Height Contours, with Lower Heights to the Left of the Wind Direction in the NH. Closer the Spacing Between the Height Contours- The Faster the Geostrophic Wind Speed.
Do Rocks Always Roll Downhill? Gedzelman, p 247 Upper-Level Winds PGF Not if the Hill is Tall Enough!
Key Concepts for Today Rotation of Earth Accelerated Frame of Reference Introduce Coriolis “Force” Apparent Force to Account for Deflection Depends on Rotation, Latitude, Wind Speed Geostrophic Balance and Wind Balance Between PGF and Coriolis Force Geostrophic Wind Blows Parallel to Contours About One Day Required to Reach Balance
Assignment for Next Lecture Topic – Centripetal force due to curved flow Frictional force near the ground Reading - Ahrens pg 155-158 Problems - 6.23, 24
Accelerated Frame of Reference You are inside a stationary car on a frictionless floor. What happens if the car begins to accelerate? Inside the car, it appears that a mysterious force is attracting you to the back wall of the box car. Your frame of reference, the close box car, is accelerating. Someone outside the car sees the car is accelerating. A force is accelerating the box car, but you are not accelerating. Their frame of reference is not accelerating. Oh No! (rest) time
Coriolis Effect- An Apparent Force Cannonball follows a straight path to an observer in space Earth rotates counter- clockwise underneath cannonball Cannonball appears to deflect to the right to an observer on earth Gedzelman, p 241