Explain Like I'm Five

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

submitted 1 month 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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[–] ComicalMayhem@lemmy.world 4 points 1 month 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 1 month 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.