# [ELI5] I honestly don’t understand the difference between centrifugal and centripetal. Help please.

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I swear my physics prof claimed one of these didn’t exist as a force – I think it was centripetal. But that was a long time ago. Maybe it was discovered recently. Such confuse.

In: Engineering

They’re the same force as viewed from two different frames of reference. Spin a rope with a weighted end and centrifugal force will have the rock pull the rope tight. But from the perspective of the rock, it’s the rope pulling on the rock. Not the other way around.

Centripetal is the force pulling you towards the center;
Centrifugal is the force you feel pulling outwards

Centripetal force is drawing toward the center, like a satellite orbiting the Earth. It keeps falling and missing the earth.

Centrifugal is the feeling of, say, being in a car turning in a circle but you feel thrown or leaning away from the center, to the outside.

Centripetal is more an actual force objectively, centrifugal force is a pseudo force subjectively.

The trick here is reference frames. And we need Newton’s laws.

Newton 1 tells us that things keep going in a straight line (or from their point of view, stay still) unless you mess with them.

To get something to go in a circle you need to accelerate it.

Newton 2 (sum of forces = mass x acceleration) then tells us to get something to go in a circle you need some overall force pointing towards the centre of the circle.

This is the *sum* of the forces, so it could be one force (as with orbits), it could be a combination of several forces (in all sorts of different directions), but overall there must be some *net* force inwards. So a centri*petal* force is any force acting inwards (the “centri” part is about the centre, and the “petal” part is about heading towards – it comes from the same Latin word we get things like “petition” from).

But what about when looking at things from the point of view of the thing accelerating?

From their perspective they’re still, and it is the rest of the universe appears to be accelerating around them. We can get Newton2 to still work by rearranging it a bit:

> F = ma

> F – ma = 0

So to get something not accelerating, we can just shift its acceleration over to the other side of the equation, and pretend that it is a force. That “force” is going to have the same magnitude as the overall centripetal force in our first case, but is going to be in the opposite direction (i.e. acting outwards!). This is our “centrifugal force” (the “fugal” part being the “fleeing from” part).

This “force” isn’t really a force in the traditional sense; there is nothing physically pushing the thing in that direction – the “actual” force is pushing the opposite way. But it can be useful to think about. It is the correction we have to make to our physics model if we want to work in a reference frame that is moving in a circle. These kinds of not-quite-forces are sometimes called “inertial forces,” “fictitious” forces or “pseudo” forces.