### Question

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

Asked by: Marcus Schultze

### Answer

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