Centripetal Force - The Real Force

The Spinning Penny demonstrates forces in motion. A force is a push or pull. To understand how and

why it works, you have to look at the forces that are acting on the penny. The shape of the balloon

makes the penny move in a circular path - otherwise the penny would want to continue to move in a

straight line. Another force to consider is friction. There's very little friction between the edge of

the penny and the balloon. More friction would cause the penny to slow down and stop.

The real force in action here is called centripetal force, which means center-seeking. This is a

force that is always directed toward the center of the circle and is actually responsible for keeping

the penny moving in a circular motion inside the balloon.

Whenever an object moves in a circular path the object is accelerating because the velocity is

constantly changing direction. All accelerations are caused by a net force acting on an object (the

force does not equal “0”). In the case of an object moving in a circular path, the net force is a

special force called the centripetal force (not centrifugal!). So a centripetal force is a center

seeking force which means that the force is always directed toward the center of the

circle. Without this force, an object will simply continue moving in straight line motion.

Imagine swinging a rope with a mass attached to the end, around in a circle above your head, much

like your cup of water on the plate. The force of tension from the rope is what provides the

required centripetal force needed to keep the ball in the circular path. The smaller the mass, the

smaller the centripetal force you will have to apply to the rope. Smaller masses require smaller

centripetal force. The smaller the velocity of the object, the less centripetal force you will have to

apply. The smaller the length of rope (radius), the more centripetal force you will have to apply to

the rope.

Centripetal force (F) and the centripetal acceleration (A) are always pointing in the same direction.

(the force causes the acceleration) If you let go of the rope (or the rope breaks) the object will no

longer be kept in that circular path and it will be free to continue on with its current velocity. It

will go flying off!

The formula for centripetal force is F = mv2/r where m represents the mass of the object, v is

the speed (magnitude of the velocity) and r is the radius from the center of the circle to the

object. A centripetal force ends up being a net force and a net force always causes an acceleration

in the direction of the net force. So if the force is center seeking (centripetal) then the

acceleration is also centripetal. The formula for centripetal acceleration is a = v2/r. [Notice that

if you multiply this by mass (m) you get the formula for centripetal force...that's because a net

force is equal to mass times acceleration.]

Centrifugal Force - The False Force

An incorrect word has worked its way into our daily vocabulary, and with it, an incorrect

understanding of the way physics works. "Centrifugal Force" ( Latin for "center fleeing") is often

used to describe why mud gets spun off a spinning tire, or water gets pushed out of the clothes

during the spin dry cycle of your washer. It is also used to describe why we tend to slide to the

outer side of a car going around a curve. It is a common explanation...the only problem is all of it is

absolutely wrong!!! Centrifugal force does not exist...there is no such thing.

Take for example this common situation. You are riding in a car going around a curve. Sitting on

your dashboard is a CD (used to be called a tape). As you go around the curve, the CD moves to

outside edge of the car. Because you don't want to blame it on ghosts, you say "centrifugal force

pushed the tape across the dashboard."--wwrroonngg!! When we view this situation from above the

car, we get a better view of what is really happening. There is enough static friction on the sides

of the tires to act as centripetal force which forces the car to stay in the circular path. The CD on

the slippery dashboard does not have enough friction to act as a centripetal force, so in the

absence of a centripetal force the CD follows a straight line of motion.

The car literally turns out from underneath the CD, but from the passenger's point of view it looks

as though something pushed the tape across the dashboard. If the car you are riding in has the

windows rolled down, then the CD will leave the car (or does the car leave the CD?) as it follows its

straight line path. Objects tend to NOT change motion unless a force acts on it and in this case,

not enough friction was acting on the CD. If the windows are rolled up, then the window will deliver

a centripetal force to the CD and keep it in a circular path. Any time the word Centrifugal Force is

used, what is really being described is a Lack-of-Centripetal Force.