CENTRIFUGAL FORCE

-a centripetal force must be applied to make an object deviate from its natural tendency to move along a straight line.

-for example the muscles of a discus thrower apply the centripetal force needed to make the discus move in a circle until the moment it is released

-one can think of the centripetal force as the opposite of a centrifugal force that is apparently pulling the object outward from the desired path

-If a string tied to a rock is swung around, the rock describes a circular path of constant radius. If we cut the string the rock flies off in a straight line. This is an illustration of Newton's first law--an object at rest or in straight line motion remains in that state unless acted on by a net force. Before we cut the string, the string exerted a force on the rock by confining it to a circular path. This force is directed inward, toward the center of the circular path, and for this reason is known as the centripetal force.

Forces bring about acceleration. We usually think of an acceleration as merely a change in the speed of an object as when an auto speeds up or slows down. However, an acceleration may also consist of a change in the direction of an object without any change in the speed. This is the case in our example of the rock on a string. The centripetal force is a net force that is responsible for a continuous change in direction and not for a change in speed. Cutting the string eliminates the centripetal force and the rock follows a straight path.

Another of Newton's laws applied to our orbiting rock on a string is the third law which holds that for every force, there is an equal and opposite force. You lean against a wall and the wall pushes back with the same force. The centripetal force exerted on the rock by the string is opposed by an equal force exerted on the string by the rock. This outward directed force is called the centrifugal force.

-air parcels in the atmosphere will experience a centripetal acceleration whenever they travel along paths that are curved relative to the earth's surface. This will occur when the isobars are curved. They may not be changing speed but they will be changing direction.

-in airflow having gentle curvature the centrifugal force is usually insignificant compared to other forces but in small circulations such as tornadoes it is very strong.

-formula

R= V(2)/r times n where v is wind speed r is radius of

curvature and n is normal outward wind direction

CENTRIFUGAL ACCELERATION

Directed outward from high and low pressure and therefore is same

direction as pressure gradient around hi and opposite direction

around low

Balance between pressure gradient and centrifugal acceleration yields cyclostrophic flow which is critical in tornadoes

NONGEOSTROPHIC WIND AND GRADIENT FLOW

Strictly speaking, air particles constrained to move in curved paths can do so only in response to a centripetal acceleration directed toward the center of curvature

However the convenience of dealing with balanced forces can be retained by recognizing that the net centrally directed force can be equated to the centrifugal force which is the reaction to centripetal force

-this flow is no longer geostrophic but rather gradient

Around low pressure

-balance of forces is between the sum of the centrifugal

and coriolis force versus the pressure gradient force

-the combined centrifugal and coriolis force exceed

the pressure gradient force

Around high pressure

-centripetal force is directed toward the center of high

but the pressure gradient force is directed outward

and coriolis force inward

-balance of forces is achieved between coriolis and sum

of centrifugal and pressure gradient force which exceed

the coriolis

GRADIENT WIND

When straight geostrophic winds approach a trough cyclone, curvature causes a centrifugal acceleration in the same direction as coriolis

-if wind speed is reduced immediately result is smaller coriolis acceleration with sum of centrifugal and coriolis acc. Balancing the pg acceleration. --i.e.isobars get closer together and gradient wind is less than geostrophic wind

-as air approaches a ridge centrifugal acceleration is in the same direction as pg acceleration. Thus air parcel must increase speed and hence coriolis acc. to achieve a balance. Result is that the gradient wind is faster than would be the case if it were geostrophic

Gradient wind is like a geostrophic wind in that is is large scale, horizontal, frictionless and parallel to isobars

-major difference is that the path is curved

Gradient wind usually develops above surface and has many characteristics in common with the geostrophic wind. It also is a large scale, horizontal, frictionless and parallel to isobars. The important difference between the two winds is that the geostrophic wind blows in a straight path, whereas the path of the gradient wind is curved. The gradient wind is not the consequence of balanced forces. Recall that the centripetal force changes only the direction of an air parcel and not the parcel's speed. the horizontal pressure gradient force, the coriolis effect and the centripetal force thus interact in the gradient wind....

A gradient wind develops at altitudes above the friction layer around a dome of high air pressure called an anticyclone or around a center of low pressure. In an ideal anticyclone, isobars form a series of concentric circles about the location of highest air pressure. The horizontal pressure gradient force is directed radially outward, away from the center of the high. the coriolis force is directed inward. The coriolis effect is slightly greater than the pressure gradient force with the difference giving rise to the inward directed centripetal force. I.E. the centripetal force results from an imbalance of other forces.

In an ideal cyclone, isobars form a series of concentric circles about the location of lowest air pressure. The horizontal pressure gradient force is directed inward and the coriolis effect is directed radially outward from the center of the low. the pressure gradient force is slightly greater than the c coriolis force with the difference equal to the net inward directed centripetal force

Is there a relationship between coriolis force and centrifugal force???

The centrifugal force is quite different from the Coriolis force. The centrifugual force is a result of objects wanting to travel in straight lines when no forces are acting on them. In

order to make objects move in circular paths, an inward-directed force (the *centripetal* acceleration) must be applied (as in whirling something tied to a string ... one must pull inward to make it go in a circle). In the coordinate frame traveling with the rotating object, however, this "feels" like a force (a *centrifugal* force) wanting to sling the object

*outward*. Centrifugal and Coriolis forces share one thing in common: understanding them requires one to be able to view processes in different coordinate systems. This stuff can get pretty deep. Centrifugal forces are in fact an issue in the weather ... any rotating flow experiences centrifugal effects, as in flow around troughs and ridges.

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