Explain Like I'm Five

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How is centrifugal force a ficticious force? (lemmy.world)

submitted 2 months ago by ComicalMayhem@lemmy.world to c/explainlikeimfive@lemmy.world

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I understand that in order for an object to maintain circular motion, its velocity vector must be travelling perpendicular to its position vector and constantly changing inwards, hence an acceleration towards the center of the circle. I know that the acceleration towards the center is typically caused by other forces, like tension on a string, and that these are called centripetal forces I believe? However, objects in circular motion tend to want to be away from the center instead of towards. A bucket of water tied to a string and twirled around in a circle will result in the water staying in the bucket: if the water is exhibiting circular motion, would it not thusly be accelerating inward, and thus escaping the bucket? I've heard that it's a difference of frame of reference, but even looking from out to in, I can't see how the water would be accelerating inward and yet remain in the bucket without support. Would there not be some force pushing the water into the bucket? And yet, centrifugal force is considered a fictitious force. I don't understand. I know I understand some level of physics but please explain it like I'm 5 because I can't seem to actually understand this.

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[–] voracitude@lemmy.world 17 points 2 months ago* (last edited 2 months ago) (1 children)

It's called a fictitious force because there's not actually a universal phenomenon like gravity or electromagnetism that causes it. When the bucket starts moving, the water is at rest, and the bucket's motion causes the water to move in a single direction. When the bucket's vector starts to change, the water's vector does not. The water is still moving in the original direction, but that direction is now "outwards" from the centre, and the bucket exhibits new motion on the water, changing the direction it's traveling again. The water is trying to ~~escape the bucket, but~~ keep going in the direction it's already going (outwards), not towards the centre of the circle the bucket is tracing. The bucket however is in the way, redirecting it along the circular path. Remove the bucket, and the water goes flying off away from the center of the circle.

Another way to understand a fictitious force is to think of a bunch of marbles in a box, packed tight so all the marbles are touching and none of them can move. Each marble is a particle, push on one and you affect others around it. Now, remove a marble so you have a space, and only one marble can move at a time. And now, reverse your vision - the space left by removing the marble is the particle, an "anti-particle", and the marbles are free space. Push on the marbles, and the anti-particle moves around like a marble would. It's not a marble of course, it's a space, so you could call this "anti-particle" you just created "fictitious".

So it's not that the water in the bucket is accelerating inwards, because it's not the water maintaining that circular motion at all - it's the bucket maintaining the circular motion. The water's "inwards acceleration" that keeps it moving in a circle is actually just the water's inertia, relative to the motion of the bucket.

[–] ComicalMayhem@lemmy.world 4 points 2 months ago (1 children)

Let me see if I understand this. So the bucket acts on the water, pushing it in a direction, then the bucket's motion changes, changing the motion of the water, then this repeats ad naseum in a circle. So the bucket is experiencing a centripetal force (tension from the string), but not the water: it's motion is changing as a result of the bucket pushing on it. So then if the bucket is moving in a counter clockwise direction, the "left" wall of the bucket would be the thing acting on the water. Wouldn't that cause the water to stick to the left wall of the bucket, not the bottom?

In regards to your example with the marbles and anti-particles, I understand it in principle, but I'm not quite sure I get how fictitious anti particles relate to fictitious forces. I mean, I think I get it, and I understand what you mean by it not having a universal phenomenon driving the force. I'm just not sure I could explain it back to you.

[–] voracitude@lemmy.world 2 points 2 months ago

So then if the bucket is moving in a counter clockwise direction, the “left” wall of the bucket would be the thing acting on the water. Wouldn’t that cause the water to stick to the left wall of the bucket, not the bottom?

It might help to remember that "left" and "bottom" are different depending on where you're standing. The water keeps moving in the direction the original force got it moving, but as the bucket traces the circular path, the "left" and "bottom" walls are now in different positions relative to the water. Remember, you don't see the water sitting peacefully on the "bottom" of the bucket like you would if it were standing at rest; the water is forced in the "bottom left corner" of the bucket while it spins, because the water always being accelerated (by the bucket) tangent to the direction its already moving.

Also, the bucket is absolutely experiencing the force of the water acting on it - right from the start of the string pulling taut, the water has been pushing on the bucket's walls (and even before, actually, the whole time the water is in the bucket). But that's not really relevant.

I’m not quite sure I get how fictitious anti particles relate to fictitious forces.

Sorry, I didn't mean to muddy the water - I was just trying to illustrate why it's called "fictitious". It looks and acts like a force (or a particle, in the case of the marble), but it isn't one because it doesn't actually exist outside that frame of reference. Centripetal force is the "pull" towards the centre of the circle that is required for circular motion; if you think of the actual motion of swinging a bucket around on a string, you'll be pulling on the string (if you don't pull, the bucket flies off in a straight line, along with the water in it). And, if you don't pull the string, and the bucket and water fly off in a straight line, then the water (and bucket) no longer experience the centrifugal force they got while tracing the circular path.