Centrifugal Force

Hey, do I have a question for you?! What do you think keeps the moon from falling into the earth - what keeps it up there in the sky?

We're going to look at some very important "Forces" that have to do with spinning objects like flywheels, crankshafts and other things on a racecar - as well as what's hanging above us.

Before we talk about CENTRIFUGAL force, it's important to understand the exact opposite: CENTRIPETAL force. Here's how it works: Objects that move in a circle are undergoing a continual change in direction (they never travel in a straight line). This means that objects are constantly being accelerated - always toward the center of rotation. Check out Figure 1 below.

The line shows the circular path of a ball moving around a center of rotation. What causes this "centripetal acceleration" are the molecular forces of cohesion within the string. The arrows show how the ball is constantly changing direction.

Molecular Forces of Cohesion: Everything that exists is made up of atoms. Different kinds of atoms can combine to form molecules. For example, H2O is the molecule of water - made up of two atoms of Hydrogen and one atom of Oxygen. Forces of attraction between like molecules are called cohesive forces. It is this molecular attraction that gives a solid (such a steel bar) its strength. (For more on this, please see the lesson on Chemistry Basics).

What happens if we spin the ball too fast? Arrow A shows the direction in which CENTRIPETAL force acts. As long as a CENTRIFUGAL force (Arrow B) is equal to the CENTRIPETAL force, everything is fine (in equilibrium). But if the CENTRIFUGAL force becomes greater than the CENTRIPETAL force, the string will break! Why would this happen?

You may have had this experience on the playground carousel. The faster the carousel turns, the harder it is to hang on! This shows the relationship between CENTRIPETAL and CENTRIFUGAL force.

CENTRIPETAL force was how hard your arms pulled on one another to hold on. CENTRIFUGAL force was trying to pull your hands apart. At some point, the CENTRIFUGAL force overpowered the CENTRIPETAL force, and stick boy took a header off the carousel! Note that the CENTRIPETAL force acts towards the center of the carousel and the CENTRIFUGAL force acts away from the center of the carousel.

This same action occurs in all materials, such as flywheels, tires and all of the belt pulleys on your racecar! The molecular structure of the material must be strong enough to supply the CENTRIPETAL force need to match the CENTRIFUGAL force created by high rotational speeds. If the molecular structure is too weak, a flywheel will break apart. This is why most racing sanctioning bodies require "scatter shields" on some high-powered cars with high-revving engines!

The figure above shows two steel balls connected to a shaft by springs. When the shaft rotates, the steel balls will move outward from the shaft. This is CENTRIFUGAL force in action and is basically how a CENTRIFUGAL clutch works (see below). It's also how the inside of some engine ignition distributors mechanical advance mechanism works. The faster the distributor turns, the weights move out and change the ignition advance of the engine. We'll learn more about this in the auto ignition systems lessons. Check out the picture on the right.

The figure above shows a simple illustration of a CENTRIFUGAL clutch like those used on racing karts. The hub is connected to a chain sprocket. The weights are connected to a shaft that is connected to the engine. When the engine spins fast enough, the shoes move out against the hub. Friction between the shoes and hub makes the hub start turning. You can tune the clutch by changing the strength of the springs or weights of the shoes. Hey, did you know that this same kind of clutch tuning is used on Top Fuel Dragsters and Funny Cars? Yep, the crew chiefs on those high-powered hot rods know all about CENTRIFUGAL force and how to use that knowledge to win races!

If you read the lesson on FRICTION, you know that the weight of the car and the tire compound determine how a car "sticks" to the road when going around a bend. This FRICTION actually supplies the CENTRIPETAL force to keep the car on the road. If we loose traction, the CENTRIPETAL force goes away - and CENTRIFUGAL force allows us to slide of the road.

So what if we want to build a track where the cars can go real fast around curves? We can bank the racetrack! This helps supply more CENTRIPETAL force on the curves. The degree of banking will determine how fast we can go and still stay on the track.

One thing we need to mention here. Just because a flywheel (or any other spinning object) can withstand a sustained RPM (spinning speed), it's not the only thing we need to know. We also need to know if it can withstand accelerating from stop to top speed without flying apart.

So what keeps the moon in constant orbit around the earth?

Hey, would you believe its the CENTRIPETAL force of gravity between the earth and moon - and the CENTRIFUGAL force caused by the speed it's spinning around the earth?

Well, that's a look at CENTRIPETAL and CENTRIFUGAL forces. Now think about all the things that spin on a racecar. Each component has to be designed to keep the CENTRIFUGAL forces from overcoming the CENTRIPETAL forces that holds them together! And now you know why!