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DA693
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Why does a body experience a fictitious force when placed over an object which is accelerating ? I mean why it doesn't just accelerate with an object... What's actually happening?
It doesn't. It just keeps doing what it's doing. Howevet, if you attach yourself to the accelerating object and wish to claim that you are stationary then you need to explain why the body is accelerating - and that's what so called fictitious forces (probably better called inertial forces) do.DA693 said:Why does a body experience a fictitious force when placed over an object which is accelerating ?
Why would it, unless it's stuck to the object or otherwise being pulled along by it?DA693 said:I mean why it doesn't just accelerate with an object
If I am stationary and the object on which I am is moving with an acceleration then what will happen it's obvious i will experience a force against that motion and like someone pulled me backwards .. Now my question is why?Ibix said:It doesn't. It just keeps doing what it's doing. Howevet, if you attach yourself to the accelerating object and wish to claim that you are stationary then you need to explain why the body is accelerating - and that's what so called fictitious forces (probably better called inertial forces) do.
Why would it, unless it's stuck to the object or otherwise being pulled along by it?
Work in an inertial frame. You are standing on a carpet and someone pulls it forwards. There is a frictional force between your feet and the carpet so your feet move as well. There is no backwards force - you fall over because your feet are no longer under you.DA693 said:If I am stationary and the object on which I am is moving with an acceleration then what will happen it's obvious i will experience a force against that motion and like someone pulled me backwards ..
Imagine this. Your friend is driving a car and you are on the passenger's side of the front seat. A book is on the back seat. The car's speedometer reads 30 mph and the car is traveling in a straight line. Your friend asks you if the book is moving, you look back and you answer "no it's just sitting there". Farther down the road your friend makes a sharp left turn. While the car is turning, your friend asks you again if the book is moving. You look back and you answer "sure, I see it sliding across the seat to the right."DA693 said:Your ans is nice but i don't get the line where you said... "wish to claim that you are stationary then you need to explain why the body is accelerating - and that's what so called fictitious forces (probably better called inertial forces) do."
Fictitious forces cannot be experienced. They can only be inferred from the motion. There is no experiment or measurement that depends on the fictitious force because they are not physical, they are just mathematical artifacts of using certain coordinate systems.DA693 said:experience a fictitious force
This is true in technical speech, but I think it can potentially be a bit confusing. When a bus stops suddenly I would certainly say that my personal experience is that a gravity-like force suddenly pitches me onto my face (some of the bus drivers round here treat bus stops like they're dropping troops into a hot LZ). I guess I'm drawing a distinction between the technical sense of "experience a force", which you can't be experiencing if you can make it go away by a frame choice, and the more mundane sense of "experience" as "my personal impression of what happened".Dale said:Fictitious forces cannot be experienced. They can only be inferred from the motion.
That is a good point. Such experiences are just illusions, much like an optical illusion. Your brain takes an actual raw sensation and changes it into something that it isn’t.Ibix said:This is true in technical speech, but I think it can potentially be a bit confusing. When a bus stops suddenly I would certainly say that my personal experience is that a gravity-like force suddenly pitches me onto my face (some of the bus drivers round here treat bus stops like they're dropping troops into a hot LZ). I guess I'm drawing a distinction between the technical sense of "experience a force", which you can't be experiencing if you can make it go away by a frame choice, and the more mundane sense of "experience" as "my personal impression of what happened".
Although you are not being accelerated.Ibix said:When a bus stops suddenly I would certainly say that my personal experience is that a gravity-like force suddenly pitches me onto my face
This is an interesting point. Have you been sitting at a stoplight waiting for it to turn green and out of the corner of your eye you notice a car moving forward and you feel a sensation of moving backward and you automatically push on the brake pedal too?Dale said:That is a good point. Such experiences are just illusions, much like an optical illusion. Your brain takes an actual raw sensation and changes it into something that it isn’t.
That is an optical illusion, as mentioned above. No actual measurement would corroborate the feelinggleem said:Although you are not being accelerated.This is an interesting point. Have you been sitting at a stoplight waiting for it to turn green and out of the corner of your eye you notice a car moving forward and you feel a sensation of moving backward and you automatically push on the brake pedal too?
Define "experience a force". Are you talking about your subjective interpretation?DA693 said:...i will experience a force ...
Optical illusion? During their training , astronauts are put into a centrifuge, and experiment accelerations many times greater than g . Centrifuges are used in chemical laboratories to separate heavier particles. Centrifuges are used in washing machines.Dale said:That is an optical illusion, as mentioned above. No actual measurement would corroborate the feeling
Intuition must be put aside. But it isn’t mathematics only, it’s physics. An astronaut experiences greater blood pressure in his brain, he can loose senses.vanhees71 said:To make the occurring equation more intuitive, one brings all the terms in ma which are different from mx¨j′ to the left-hand side. Then the EoM. look like a Newtonian equation of motion, and one calls the additional terms "inertial forces". That is just mathematics and makes the phenomena occurring in non-inertial reference frames more intuitive, i.e., you have in addition to the "true forces", due to the fundmental interactions between particles, these non-inertial forces
AS you well know, non inertial forces are considered in non-inertial r.f. only .vanhees71 said:You can describe the astronaut as seen from the inertial frame outside the spaceship he's accelerated against this inertial frame, and then the raise of his blood pressure is clearly due to the "true forces" needed to accelerate him.
But not because of inertial forces. Inertial forces can be transformed away via a coordinate change, while the pressure gradient is frame invariant. Explaining frame invariant effects with frame dependent forces makes no sense, because they are already fully explained by the frame invariant forces.italicus said:An astronaut experiences greater blood pressure in his brain,
italicus said:But it isn’t mathematics only, it’s physics.
Yes, this is the only effect of the inertial force: to explain the coordinate acceleration in the non-inertial frame. All other effects are due exclusively to the real forces.italicus said:If you want to apply Newton’s law of motion : "F = ma” In an accelerated reference frame , you have to add inertial forces to the real applied force F . This gives the acceleration relative to the non inertial reference frame.
Yes, and this is due entirely to the real centripetal force. Consider this: where is the blood pressure greatest and hence where are vessels most likely to rupture?italicus said:An astronaut experiences greater blood pressure in his brain
You need to be careful what you say to students. It is a fine line between simplifying things to aid in learning of a concept and teaching wrong things that will have to be unlearned later. I don’t know what you are saying, but if you are attributing a real physical effect like the blood pressure to a fictitious force then you are teaching a wrong thing.italicus said:This is the way I teach the problem to students.
robphy said:Watch the Frame of Reference video (Ivey & Hume, 1960)
- time-stamp 00:33: the intro is a classic (a must-watch clip)
- time stamp 13:27:
where they begin to discuss non-inertial frames of reference (but you may need to see the preceding sections for the inertial-frame)...- punchline: 16:10
- rotating-frame: 17:04
Don’t worry, I am very careful. After having introduced the issue, I receive a lot of questions, to which I start answering with great patience and details, and it takes a lot to arrive at the right conclusion. I am accustomed to discussing with people.Dale said:You need to be careful what you say to students. It is a fine line between simplifying things to aid in learning of a concept and teaching wrong things that will have to be unlearned later. I don’t know what you are saying, but if you are attributing a real physical effect like the blood pressure to a fictitious force then you are teaching a wrong thing.
The centripetal force is directed towards the axis of rotation.Dale said:Now, going back to the centrifuge, the forward-directed force is the real centripetal force.
Yes, which is the correct (forward) direction to explain the blood pressure.italicus said:The centripetal force is directed towards the axis of rotation.
Exactly, and it's always easier to understand a situation wrt. an inertial reference frame!italicus said:AS you well know, non inertial forces are considered in non-inertial r.f. only .
Bond is of course correct here, regarding what will crush his bones: The real centripetal contact force. Just like the real contact force of a bus hitting you is what kills you. This real contact force exists in the rotating frame as well, so no additional explanation via inertial forces is needed for Bond's demise.kuruman said:I think that this xkcd cartoon summarizes it well.
Sorry but this is an idea of yours. Having well in mind that the real force is the centripetal one , consider that , more o less, we live in a rotating reference frame, the one of the Earth, and say that the acceleration of gravity , in this rotating frame to which we belong, is less at the equator than at the poles. And the Coriolis force, acting on air masses which moves from the equator to North Pole , deviates them to the right, ok ? The same deviation applies to air masses that move from the NP to the equator : to the right . This causes hurricanes ( Katia...) with a counterclockwise sense of rotation , if seen from “above” , e.g. from meteorological satellites , no ? (I have simplified the real situation, because also centrifugal ( ops!) force is to be considered, and pressure variations too...but let’s be simple). So I think, ( and you can contest of course) that the description of these phenomena is as good from the inertial point of view as from the non inertial point of view.vanhees71 said:Exactly, and it's always easier to understand a situation wrt. an inertial reference frame!
The last lines could have been "Do you expect me to torque?", "No, Mister Bond, I expect you to die!"kuruman said:We haven't from the OP in quite a while and it is not clear whether the ongoing conversation addresses OP's original question. I think that this xkcd cartoon summarizes it well. Sorry for this touch of levity.
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This is correct. To explain the motion in the non-inertial frame you introduce the inertial forces. This is the only effect of the inertial forces.italicus said:you can also describe motion from the point of view of a fly, which is attached to the rod. And in the rotating frame you have to introduce the centrifugal force, don’t you?
This homework problem was a lot easier to solve in the rotating reference frame using the centrifugal force:vanhees71 said:Exactly, and it's always easier to understand a situation wrt. an inertial reference frame!
Ibix said:Work in an inertial frame. You are standing on a carpet and someone pulls it forwards. There is a frictional force between your feet and the carpet so your feet move as well. There is no backwards force - you fall over because your feet are no longer under you.
Now work in the non-inertial frame where the carpet is stationary. In this frame, when the carpet is pulled we want it to stay stationary, but if it had a net force on it then it would accelerate. So we need to invoke an inertial force to explain why the carpet doesn't accelerate, and that inertial force accelerates everything backwards at exactly the right rate to cancel the acceleration of the carpet. But you have no other force on you so you do accelerate backwards. The important thing to realize is that inertial forces are just a way of explaining why an accelerating reference object is stationary. You can always choose a non-accelerating reference object and they disappear.
Ibix said:Work in an inertial frame. You are standing on a carpet and someone pulls it forwards. There is a frictional force between your feet and the carpet so your feet move as well. There is no backwards force - you fall over because your feet are no longer under you.
Now work in the non-inertial frame where the carpet is stationary. In this frame, when the carpet is pulled we want it to stay stationary, but if it had a net force on it then it would accelerate. So we need to invoke an inertial force to explain why the carpet doesn't accelerate, and that inertial force accelerates everything backwards at exactly the right rate to cancel the acceleration of the carpet. But you have no other force on you so you do accelerate backwards. The important thing to realize is that inertial forces are just a way of explaining why an accelerating reference object is stationary. You can always choose a non-accelerating reference object and they disappear.
YrA.T. said:Define "experience a force". Are you talking about your subjective interpretatio
A.T. said:Define "experience a force". Are you talking about your subjective interpretation?
A body experiences fictitious force on accelerating objects because of the frame of reference in which it is observed. In non-inertial frames of reference, objects appear to experience forces even when there is no actual force acting on them. This is due to the fact that the frame of reference itself is accelerating, making it seem as though there is a force acting on the object.
Fictitious force and real force are different in that fictitious force is only observed in non-inertial frames of reference, while real force is present in both inertial and non-inertial frames. Fictitious force is a result of the frame of reference itself, while real force is caused by an external agent acting on an object.
Newton's second law, which states that force equals mass times acceleration, still applies to fictitious force. However, since fictitious force is not a real force, it does not actually cause acceleration in the object. Instead, it is a mathematical construct used to explain the observed motion of objects in non-inertial frames.
No, fictitious force cannot be eliminated because it is a consequence of the frame of reference in which an object is observed. However, it can be accounted for and mathematically canceled out in calculations by using the appropriate equations for non-inertial frames.
The Coriolis effect is a specific type of fictitious force that is observed in rotating frames of reference. It is responsible for the apparent deflection of moving objects on the surface of the Earth, such as winds and ocean currents. The Coriolis effect is caused by the rotation of the Earth, which creates a non-inertial frame of reference for objects on its surface.