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How is centrifugal force a ficticious force? (lemmy.world)
submitted 1 year ago by 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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[–] 3 points 1 year ago* (4 children)

Like you said, objects in a circular motion want to be away from the center of a circle instead of towards it. Centrifugal force is the term to describe that specifically. It's a "made up" force, because there is no force pushing an object away from the circle.

Q: How is there no force pushing an object away from the circle?

A: An object moving in a circular motion is at all times already trying to move away from the circle. If you take the bucket of water for example, and suddenly deleted the bucket, the water would keep flying in the straight line it was trying to go in. The direction it would fly would be sideways, perpendicular to a line drawn to the center of the circle and not outward away from the center.

Q: Is the water accelerating inward?

A: Yes! The bucket pushes the water keeping it from going in a straight line. Likewise, the string pulls the bucket which keeps the bucket from flying out. And you are spending the energy to cause the force that is being applied to the string. This is known as "centripetal force". It's the force that makes the circle going objects change direction.

Q: If there is force or acceleration inward, why doesn't the water fly towards the center?

A: Acceleration is not a change in speed. It is a change in velocity, which is a combination of speed AND/OR direction. So the water doesn't fly inward because centripetal force only causes a change in direction (the bottom of the bucket keeps the water from going straight). If someone punched the bottom of the bucket while it was swinging in a circle, then the water would fly inward out of the bucket because that would cause a change in speed toward the bucket.

Short summary: Nothing is pushing the water into the bucket, the bucket is pushing the water so it continues to accelerate in a circle. The water wants to go straight. That is inertia.

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  • [–] 2 points 1 year ago (1 child)

    Thank you for this explanation.

    Two sentences that are a bit confusing:

    The direction it would fly would be sideways, perpendicular to a line drawn to the center of the circle and not outward away from the center.

    fly inward out of the bucket because that would cause a change in speed toward the bucket

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  • [–] [S] 1 point 1 year ago (1 child)

    Your third point clarifies some things for me a lot. I seem to have forgotten that acceleration describes a change in direction and/or a change in magnitude of the velocity vector: I recall now in my physics textbooks that objects in non-constant circular motion have a tangential acceleration, and the total acceleration lies somewhere between those, but if the velocity remains constant, then the only acceleration is the centripetal acceleration describing the change in the velocity's direction.

    However, I still have some questions about points one and two. I understand that things in circular motion want to fly out tangentially away from the center, not radially away. Yet, in so far as I can observe, objects do seem to press outwards radially. In the example with the bucket of water, the water sticks to the bottom of the bucket instead of pressing against the side wall. In another example, that of those carnival rides that spin people around in a saucer (gravitron I think it's called?), the carmival goers tend to stick to the wall of the ride as though they were being flung out radially, instead of rolling along the edge or something else. I guess it's this disconnect between what I know is correct (objects fly tangentially to their circular paths) and what I observe (objects stick to the wall radially away from the center).

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  • [–] 2 points 1 year ago* (last edited 1 year ago)

    That's fair. The reason it seems to stick to the bottom is because it is the bottom of the bucket causing the change in direction. Kinda confusing right?

    There is an outward force, as the bottom of the bucket pushes the water to have it change direction, the water does push on the bucket in the opposite direction (3rd law). But, this is not a "centrifugal force" which describes a force pushing the water outward. To reword, the water is making a force against the bucket in response to the bucket's force on the water, but that force is soley generated by and in response to that interaction. Ironically, this might be easier to visualize with a satellite in orbit and gravity. Gravity is pulling the object toward Earth, that's easy to understand. But, the object is also moving laterally around Earth, so it sorta is kinda in a state of constantly falling (centripetal force) and missing. Same with the water, but it's the bucket pushing and not gravity pulling.

    This might raise the question: Why does the bucket need walls to keep the water in?

    First, to get the bucket in motion, starting from rest, you do need to increase and maintain it's speed, and same with the water in it, that force is a different force to the centripetal force (though in this case the same source being your arm). On the gravitron the force to speed you up is friction which exists due to the normal force caused by the centripetal force itself. And in space it'd be like booster engines or smthn idk. Second, there is air in the way on Earth.

    I hope that helps.

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