QUESTION #150

I often get confused between the terms centrifugal force and centripetal force could you please enlighten me?

Both terms describe forces associated with circular motion, but let's start with a straight line analogy. If you are in a car whose speed is increasing, the car is being accelerated by a force applied in the direction of travel. Inside the car, you feel as though a force is pushing you back in your seat toward the rear of the car. Those two forces, one forward and one backward, are the straight line equivalents of centripetal and centrifugal force in circular motion.

If the car described above is moving at a constant speed, but changing direction as it travels around a curve, it ALSO is accelerating. Acceleration occurs anytime VELOCITY changes, and velocity is defined as a combination of speed AND direction. The car is changing direction because of a force (supplied by friction between road and tires) directed toward the center of the curve. That is the centripetal force, which is always directed toward the center of the curve. When you swing a weight around your head at the end of a string, your hand supplies the centripetal force to keep the weight moving in a circle.

Back inside the car, you feel a force pushing you AWAY from the center of the curve. That force is called centrifugal force, which is always equal in magnitude but opposite in direction from centripetal force.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

You have a lot of company in your confusion. A lot of people get centrifugal and centripetal force mixed up. I did too! Here is the way I keep them straight. 'Centripetal' comes from the Latin word for 'center seeking' so the direction that centripetal force acts in is towards the center of a circle. Think of the 'p' as the first letter of 'pushed', as in 'being pushed towards the center'.

'Centrifugal' has two Latin roots in it. You can see the same 'cnetri' from centripetal, so the idea of 'center' is a part of this word. The other Latin root is 'fugere', which means 'to flee'. So, 'centrifugal' means 'to flee from the center'. Think of the 'f' as the first letter of flee as in 'fleeing from the center of the circle'.

I presume you know that of these two, only centripetal is the real force. For something to go in a circle it must be being pushed or pulled (there's the 'p' again!) towards a center point; otherwise it would be going in a straight line. Centrifugal 'force' is really a function of the inertia of the object being pushed into a circle. It is not really a force at all, it is simply the tendency of an object to go in a straight line.
Answered by: Tom Young, Science teacher, Whitehouse High School