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.

It’s about your frame of reference.

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.

Centrifugal: When you are on a merry-go-round that is spinning fast, you feel like you are being forced outward.

Centripetal: Gravity pulls you towards earth (better explanation is the satellite falling, but i like mine for ELI5)

To memorize these in class, i used to use the P in Centripedal as a pull. And the F in Centrifugal as forcing away.

The reason why Centrifugal force is a fake force, is because, say you are in a car that is turning left really fast. You feel a strong (centrifugal) force forcing you to the right. You only feel that because the car is changing direction and your body wants to keep going in the old direction it was. Nothing is actually forcing you outward.

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