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Centripidal and centrifugal forces


RenegadeOfPhunk

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Heh - indeed Spider, this thread probably shouldn't have been ressurected. ;)

 

...but now that it has, and now that ET has gone and said this:

 

The door is accelerating you inwards, towards the center of the circle it is moving in. THIS is centripetal force, because it is actually accelerating you.

 

Your push on the car door, while you ARE exerting a force, you are NOT accelerating the door, and therefore it is NOT centrifugal force

 

I feel I must retort.

Heh, ok. Nice try ET ;)

 

Your choosing to imply that because the centrifugal force isn't actually causing 'centre-fleeing' movement, that means you can't actually call it centre-fleeing.

 

...you choice to see it that way I guess...

 

If the door mechanism were to fail, or the guy in the car were a particularly fat b**tard, then the door could very well end up being forced away from the centre of movement (by being ripped from it's hinges), and nothing would have changed about the basic nature of the force involved, only it's magnitude or the body the force is being exerted against.

 

...so why you would suddenly decide this force requires a different name just because the door happens to be bolted to the car is beyond me.

I think you can glean a clue from the word centripital - meaning centre-seeking. Notice the 'seeking' bit. It seeks to get to the centre. Doesn't mean it's going to actually reach the centre, it just means it WANTS to get to the centre.

...and since centrifugal is the opposite of centripital, this means that objects having a centrifugal force applied to them WANT to get away from the centre of movement. (e.g. the door of the car) Whether they can or not is a different matter, but doesn't change the nature of the force, or what you should call that force...

 

You pull a fridge door. You don't pull hard enough to break the seal and it doesn't move an inch.

You pull again, but harder this time. The seal breaks and the door opens.

...you'd like to propose that instead of calling both instances a 'pulling' force, we call one a non-door opening force, and the other a door-opening force?! How is that making anything simpler, or helping us understand what's going on better?

..the fact is it doesn't. Most sensible people would just call that two instances of pulling a fridge door. One was big enough to open it, one wasn't. But both were instances of the same force...

 

...I think your really clutching at straws to try and keep this myth of centrifugal force being fundementally 'imaginary' intact...

 

Consider the example of the Earth orbiting the Sun. This is an example where there is NO good reason why the term 'centrifugal' force cannot be used 'correctly'. (Assuming the term 'centripital' force is still legitamete), since the Sun is INDEED moving in a centre-fleeing fashion as it is being tugged by the Earth along it's orbit...

 

 

You can quote your text books all you like. You can even drop the word centrifugal force and called it 'inertial' force if you like.

 

Potato. Potaado.

..I can choose which termanology I'm confortable with as long as it leads to correct results. Thinking in terms of centripital forces and centrifugal forces leads to correct conclusions, because it's still the same stuff your talking about - it's just using the direction of the force rather than the source. That's it.

Trying to imply there is more to it than that is missing the point entirely.

..I'm with this guy: http://www.physicsnews1.com/question_5.html

 

Centrifugal force is only imaginary in particular cases where people misunderstand the basics of physics. When used appropiately and in context, centrifugal force is perfectly real...

 

I've already swallowed humble pie over not knowing that such a concept as an 'imaginary' force existed.

...I think it's your turn to cut a slice for yourself here. Centrifugal force is only imaginary when the term is used innaccurately. End of story...

 

The only legitemate reason I can see for ditching the term centrifugal force is to keep termainology consistent, as Spider has mentioned. And I accept this...

...but it's still wrong to call all centrifugal force 'imaginary'. This has got to be one of the most astounding myths in modern physics I've ever come across...!

..and many perfectly intelligent people seem to have swallowed it whole!! :eek:

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My explanation was spot on, I take none of your pie :xp:

 

it is called centripetal force because it is causing an acceleration. It is actually accelerating you inwards. YOU are NOT causing an acceleration on the door by pushing it outwards.

 

You are exerting a normal force on the door, and the door is exerting a normal force on you. We call the normal force from the door the centripetal force because it's actually accomplishing an acceleration.

 

 

But yes, I suppose if you felt that calling your normal force on the door the centrifugal force made you feel warm at night you could always do that and still be pretty much correct in your description of the forces :D

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There is no force that goes outward in the circle. There is a force accelerating the object towards the center of a circle. The forward momentum is carrying that object in the direction based on the tangent of its current path. The resultant motion is around in a circle. There is no force that moves an object directly away from the center of rotation.

 

untitleda.GIF

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In the specific instance of the car door, were only debating what deserves to be called 'centripital' or 'centrifugal'. I just want to clarify this has nothing to do with imaginary or real -we both know this force is real, no matter what we call it.

I believe my fridge door analogy is perfectly adequate to back up my view on things, but I do accept you can see it your way too - in THIS particular case...

 

...but I've gotta be clear, are you STILL claiming that using the term centrifugal force is always going to be describing an 'imaginary' force? (This is the only point I really care about in all this and my only main point of contention with you btw...)

...because if you are, the above debate isn't gonna help you. If the door was ripped from it's hinges and then flies outwards then by your own definition you have a legit, real centrifugal force causing that motion.

And you REALLY need to tackle the point I made about the Sun and the Earth. You have NO right to call that 'centrifugal force' imaginary OR non-centrifugal, since the Sun is indeed being pulled away from the centre of the circular motion...

..sure I can't tempt you with some of this pie? ...quite tasty ;)

 

Shok_Tinoktin, trust me - your about 5 steps behind us in this debate.

When someone is pressed up against a car door -where the car is travelling in a circular motion, the door is feeling a 'real' force, and it is the EXACT opposite to centre-seeking. THis is what we are debating here.

What you decide to call it isn't relavent to the fact it exists and it's real...

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in order for a force to be the direct opposite of a "center-seeking force", it would have to be going directly outward from the center of the circle. as for the car door analogy, the door is the centripedal force, so if there was a centifugal force, and the centripedal force was removed (i.e. the door ripped off its hinges), then you would accelerate outward from the circle. Such is not the case, you would move at the speed and in the direction you were already travelling, thus no force is acting on you, thus there is no centrifugal force.

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Shok_Tinoktin,

 

Again, you are 5 steps behind in this debate...

 

...please don't make the mistake of taking me for a fool Shok - like ET did at the beginning of this thread... I know physics...

 

If the door suddenly didn't exist then yes, you would continue on your straight path beause of your already existing momentum, not because of any force being applied on you at that point.

...many people think this is down to centrifugal force. They are wrong. But I already knew this was wrong. This is a 'legitamete' example of an 'imaginary' force.

 

But imagine if there were an elephant crammed into the car. The elephant could easiely exert enough 'inertial' or 'centrifugal' force (Potato. Potaddo.) on that door to rip it off it's hinges and send it flying outwards.

...what causes that door to fly outwards? Is it the centripital force? Of course not - otherwise the door would fly INTO the car.

Is it like what we just talked about - i.e. it's own momentum? No - of course not. If this were true, the door woudl fly off EVERY TIME the car turned a corner.

 

...the door woudl be ripped off the car because of the FORCE being exerted by the body inside the car. It's acting away from the centre of the circular movement.

And this force is NOT imaginary!! Imaginary forces don't rip car doors from their hinges!!

 

QED

 

...Jesus, this is like pulling teeth... :rolleyes:

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IT IS THE MOMENTUM OF THE OBJECT INSIDE AND NOT THE DOOR! An elephant has a greater mass and thus a larger momentum. The law of the conservation of momentum says that when two objects collide, momentum is conserved, but may be transferred. A sufficient momentum being given to the door and in the direction of the movement of the object could tear the door off the hinges purely as a result of the momentum itself.

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The law of the conservation of momentum says that when two objects collide, momentum is conserved, but may be transferred. A sufficient momentum being given to the door and in the direction of the movement of the object could tear the door off the hinges purely as a result of the momentum itself.

 

Well done. You've come up with a fancy way of saying 'One object can apply a force onto another object when it slams into it'.

 

You don't seem to understand that the only thing being debated here is termaninolgy. Nothing more... So coming up with yet another way of saying the same thing isn't helping anybody...

 

The fact is my argument would be the same if somebody decided to kick the door off it's hinges (assuming they were strong enough of course). As long as the car was travelling in a circular motion, and the kick was directed away from the centre, then the force applied to the door by the kick would be a 'centrifugal' force -going by the definition of the word centrifugal i.e. 'centre-fleeing'...

..are you gonna tell me that kicking something doesn't exert a force on it? If so, please provide your definition of a force... I'd love to hear it...

 

...hell, just throw a bloody tennis ball out of the car window away from the centre of motion. Voila. You've just proved centrifugal force exists...

 

Please read the whole thread from the beginning. Perhaps that will help you understand what the actual point of contention is here...

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I am content to agree, that you can CALL it centrifugal if you want to. I simply choose not to since that was the way I was instructed, but your use of it is like if you were talking about physics in german. Sure the words aren't the same but they've all the same meanings behind them :)

 

 

maybe a little pie

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:)

 

..tell ya what, I'll have another piece if it makes you feel better mate :) It's pretty tasty stuff ;)

 

Thanks for at least acknowledging the point I've been trying to make... This isn't really about deciding who's right and who's wrong. It's - like you say - acknowledging you can actually speak the truth in two different 'languages'...

 

Telling me that all centrifugal force is imaginary is like going up to a Frenchman and saying 'Bonjour isn't correct! Hello is!'

 

(...waits for a 'Freedom Fries'-like comment ;))

 

Maybe my statement 'All centrifugal force is imaginary' is a 'myth' was a bit strong - to be fair.

 

...I just find it suspicious that all the examples I've seen of 'imaginary' centrifugal force involves something similar to sliding along a car-seat!

That doesn't explain anything, because as I've already shown in this thread, that was known to be a misnomer back when the term was accepted anyway!!

...I just don't trust that these people are explaining properly the REAL reasons for completely dismissing the term centrifugal force - in all instances...

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First off, I would like to apologise for not reading everything first. All right, now that I have read the entire thread, I think I have a simple explaination of why there is a centripedal force, but not a centrifugal force.

 

A centripedal force pulls an object in a circle, by pulling it to the center of that circle (or pushing for that matter), but a sideways velocity of some kind prevents it from hitting.

 

A centrifugal force would thus have to pull an object traveling in a circle directly away from the center of that circle. I can think of no such force, but if anyone else can I will gladly concede.

 

In examples such as the car, the force that would tear off the door, would not be in the opposite direction of the center of gravity, and I think a diagram can show this:

 

Centrifugal.GIF

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The diagram above is absolutely correct - in my way of thinking...

 

A centrifugal force would thus have to pull an object traveling in a circle directly away from the center of that circle. I can think of no such force, but if anyone else can I will gladly concede.

 

I can conclusively PROVE to you that that centrifugal force you have penciled in your diagram HAS to be just as real as the centripital force...

It's because of Newton's third law:

 

For every action, there is an equal and opposite reaction.

 

NOTE: It DOESN'T SAY:

For every action, there is an equal and opposite imaginary reaction.

..the re-action is just as 'real' as the action.

 

What I think your mistaking is that you think the centrifugal force would be acting on the same object that's being affected by the centripital force. THis is incorrect.

Let me try and make it clear.

 

Person in a car

Centripital force = Door -> Person

Centrifugal force = Person -> Door

 

Earth orbiting the Sun

Centripital force = Sun's gravity pulling the Earth

Centrifugal force = Earth's gravity pulling the Sun

 

etc. etc.

 

If there is a 'real' centripital force acting towards the centre of the circular motion, then according to Newton's third law, there MUST be an equal and opposite 'real' force acting in the opposite direction (The centrifugal force you have marked on your diagram...)

 

I know ET and perhaps yourself have a different 'way' of explaining the same thing - but here is the important point.

 

If you ask me to predict what will happen in a given situation involving circular movement using real centrifugal forces, I WILL come to the correct conclusion, assuming I do my sums right...

 

I'm sure you will also come to the right conclusion with your methods as well - involving 'imaginary' forces.

 

But I'm happy using mine thanks ;)

..if it ain't broke, don't fix it - as far as I'm concerned...

 

The bottom line here is you can't tell me that centrifugal force (when refered to properly) is imaginary, because according to my definition of centrifugal force, it's perfectly real.

...I've just proven it using your own diagram...

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I think the REAL point here is that thinking too much about it will make your head hurt :D

 

 

In Shok's diagram only TWO of those forces are actually forces, the blue and red arrows. The green arrow is just the current velocity of the object.

 

(lets pretend it's a ball on a string being swung around)

 

Now the OUTWARD force is the force of the ball pulling the string

 

The INWARD force is the force of the string pulling the ball.

 

We call these normal forces.

 

We call the INWARD force the centripetal force because it is accelerating the ball inwards.

 

We do NOT call the outward force centrifugal force because it's not actually causing an acceleration on the string.

 

HOWEVER, if you WANT to call it centrifugal I really think that's up to you, just don't expect me to know what you're talkin about :D

 

 

It's not so bad this pie, just a little bitter

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We do NOT call the outward force centrifugal force because it's not actually causing an acceleration on the string.

 

But ET - in your example something IS being accelerated outward - the thing (I assume person) swinging this ball around them!!

 

The ball (perhaps minutely, perhaps not - depending on it's mass) will be pulling the person AWAY from the centre of motion.

...this is obvious with hammer throwers. They 'wobble' as the hammer pulls them away from centre as they spin...

 

So now - explain to me exactly why I'm not correct in calling the force which causes the 'swinger' to 'wobble' centrifugal

 

* It is a 'real' force

* It causes 'real' motion

* It is acting away from the centre of the circular movement...

 

You can call it whatever you like. I don't give a rats-arse. Just please tell me why the word 'centrifugal' doesn't make sense in this instance so that I shouldn't use it...

...you haven't come CLOSE to coming up with a reasonable argument on THIS specific point I'm afraid...

 

[edit]

..at some point you HAVE to admit (without saying it and then trying to take it back again in your next post) that this is JUST terminology and NOTHING MORE.

By the definition of centrifugal force (centre-fleeing), when a hammer thrower wobbles, you can classify the force which causes that wobble as centrifugal force.

...the only way to make that not true is to alter the definition of centrifugal force (i.e. make a new language). There IS no other way...

[/edit]

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a net force (vector) in a certain direction acting on a mass (scalar) will have a resultant acceleration (vector) in that same direction. If you have a heavy object on a rope, and you swing it in circles, then let go, it will continue travelling in a straight line given the tangent of the circle at the point where the object was when the rope was released. The direction of the acceleration will be in the direction of the inertia arrow on my diagram. So, working backwards, it can be seen that the force acting on it was not travelling in the direction of the centrifugal force arrow.

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If you have a heavy object on a rope, and you swing it in circles, then let go, it will continue travelling in a straight line given the tangent of the circle at the point where the object was when the rope was released.

 

Your not keeping up with the conversation here Shok. I've already covered things like sliding across a car seat when a car turns, or when something flies off when you let go of it whilst spinning it round.

 

It has nothing to do with ANY force. It's only existing momentum generated from a previous force. I already know this perfectly well...

 

This is DIFFERENT to the REAL centrifugal force I'm talking about. Belevie me, it's perfectly real. You just have a different name for it. That's all.

 

I just believe my name is perfectly valid. I have not heard one argument to suggest my name (centrifugal force) doesn't make sense, or isn't 'real'. (When used properly of course - but that goes without saying...)

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ok, ok I think I may have a solution. I haven't thought this out to much, but bear with me.

 

When drawing a force diagram, you have to consider the objects one at a time.

 

The forces acting on the person: Normal/centripedal force.

 

The forces acting on the car: Friction/centripedal force.

 

The forces acting on the ground: Applied force.

 

The forces acting on the door: Applied force, normal/centripedal force.

 

If I am not mistaken, you are claiming that the "centrifugal" force is being applied to the door. In actuality, the applied force is in the direction of the momentum of the person, and the normal/centripedal force is a result of the car, pulling the door in a circle.

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If I am not mistaken, you are claiming that the "centrifugal" force is being applied to the door. In actuality, the applied force is in the direction of the momentum of the person, and the normal/centripedal force is a result of the car, pulling the door in a circle.

 

Right - NOW your starting to catch up Shok So maybe you can now move on to the real question...

 

Since we are agreed that this force applied by the 'momentum of the person' is a real force (not imaginary) - and not only that, but it is acting in opposition to the centre-seeking force, (as predicted by the Third Law and which by definition makes it the centre-fleeing force...)

 

...and since centrifugal means (for the umphtempth time) 'centre-fleeing' - then why for the love of God can't I call it centrifugal force?!

*rasies arms to the sky* ..why can't I call a spade a spade for crying out loud...!! :D

 

You must understand Shok, that teaching that all instances of centrigual force are 'imaginary' is a relatively modern teaching. I did A-level physics, and we were taught very differntly.

..so far, I see no reason to pick up this idea of imaginary forces. To me, it solves nothing.

 

[edit]

Here, I'll prove my point. Here is an explination of centripital and centrifugal forces in the 'old school' (and as far as I'm concerned CLEARER school) of thought...

http://www.infoplease.com/ce6/sci/A0811114.html

..notice how centrifugal force IS perfectly real (no mention of it being 'imaginary' in any way...) It is the force-pair to centripital force...

..and again - if you missed this link earlier:

http://www.physicsnews1.com/question_5.html

[/edit]

 

 

ok, ok I think I may have a solution. I haven't thought this out to much, but bear with me.

 

Believe me Shok, I already have a pefectly sound solution...

...it's to accept that centrifugal force (if you accept the literal definition of the word centrifugal of course) is perfectly real... :D

 

I don't need your solution thanks. No offense, but your welcome to it ;)

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Originally posted by RenegadeOfPhunk

Since we are agreed that this force applied by the 'momentum of the person' is a real force (not imaginary) - and not only that, but it is acting in opposition to the centre-seeking force, (as predicted by the Third Law and which by definition makes it the centre-fleeing force...)

 

We do not agree. The momentum of the person is not in and of itself a force, and not in opposition to the center-seeking force. The momentum is in the direction that is labeled as "inertia" on my previous diagram. It is pushes the door in that direction. The door, in turn, pushes back in the opposite direction. These forces are not centripedal or centrifugal forces.

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Shok, if you did not agree with my previous statement, then you do not fully understand the implications of Newton's third law, and as such, were not gonna get very far.

 

Your confusion on this matter is evident in the way you are contridicting yourself. You correctly stated that the force of the door pushing against the person is the centripital force, and yet now you say:

 

The door, in turn, pushes back in the opposite direction. These forces are not centripedal or centrifugal forces.

 

The force of the door pushing against the person IS the centripital force, and yet is NOT the centripital force?!

...huh?

...or do you believe that 2 seperate forces are acting from the door on the person?! (One centripital and one - ermm - re-actionary?!)

 

Either way, your dead wrong...

 

There is only 1 force acting from door against person. It is the centripital force caused by the movement of the car.

There is also only 1 counter-force acting in the opposite direction (predicted by Newton's law). It is acting between the person and the door caused by the person's momentum.

I call it 'centrifugal force'. You call it the force caused by their momentum.

 

You using the 'source' of the force to define it's name. I'm using the 'direction' of the force to define it's name.

..and here you are thinking you know more than I do :rolleyes: I can only pity you at this point...

 

But it's ok. Believe centrifugal force isn't real.

I'm truly past caring now. You can only bash your head against a brick wall so many times before you find something better to do... :rolleyes:

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The car is what is exerting the centripedal force, via friction. The door supplies a normal force in the direction opposite of the momentum, which is not directly away from or towards the center of the circle. However, I too am beyond caring, but I find it hard to walk away form a challenge, so unless you stop trying or convince me, I'll probably keep at it.

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The door supplies a normal force in the direction opposite of the momentum, which is not directly away from or towards the center of the circle.

 

Aha - so you think there is a re-actionary force - opposed to the direction of momentum! which is acting perpendicular or at some other offset to directly to OR away from the centre...

 

I understand where your coming from now. THe only problem is your wrong. What your probably getting confused with is the direction of the initial inertia

...but of course this isn't a force :) ..neither can it create a re-action force I'm afraid... Only a real force (not inertia or acceleration or blah blah) can be matched with a real counter-force.

 

This is basic stuff concerning Newton's third law, and stuff you need to grasp before you will understand what's going on here.

...and if ET is gonna be objective, I hope he'll correct you on this too...

 

Read up some more on the details of circular motion. In fact look at your own diagram ;)

The only thing on your diagram (which is correct btw - I guess you copied it out of a textbook, even though evidently you didn't quite understand it...) which is not pointing either directly towards the centre of motion or away from the centre of motion is the intial 'inertia'. (If this initial inertia wasn't present, there would be no circular motion - the object would just travel straight towards the 'source')

 

But surely you should know that inertia isn't a force. (It is created by force and can create a force if it impacts into something else, but not while it is happily trundling freely along it's circular path... ;) )

 

You've spent several posts thinking you had to teach me this (with your letting-go of swinging objects and what-not), when I already knew it! Now it looks like I have to remind you of this fact.

...in your diagram of 'classic' circular motion, the only forces at work are pointing into or away from the centre. There are NONE, either action OR re-action that are pointing in any other direction...

 

Re-read your text book. And more carefully this time...

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It came out of my head. I was not drawing it as a force, it was only the direction that the momentum was moving. The force involved is an effect of the collision, in which an applied force is pushing the door in the direction of the momentum. I only labeled that as inertia, because that was a term that was being used, and I thought I could keep it simples like that. Perhaps a diagram of the forces between the door and the person is in order:

 

Door.GIF

 

Since the car is pulling both the person and the door together around the circle, neither one is exerting a centripedal force on the other. Relative to each other, this is a static problem. Therefore, the net force equals zero. The only force that the person exerts on the door is an applied force that is a result of, and in the direction of its momentum. The door applies a force on the person that is perpendicular to its surface, and equal in magnitude to the component of the force that is pushing against it. Depending on the angle, there may be more unnaccounted for force, which is opposed by a force of friction.

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Well, thanks for making what you believe is happenning clearer.

Unfortunately, what you beleive is happenning doesn't match up with reality.

 

Please read up in your physics books. Once you come to the realisation that the only forces involved in circular motion are directed exactly towards (and away from -the counter-force) the exact centre of the motion (and in NO other direction) then we can try and talk this through further. Till then, anything I say isn't gonna make any sense to you I'm afraid...

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What I am saying, is that what you are saying is not what is causing the circular motion. I was describing what is going on between the door and the person, which has nothing to do with the circular motion. Surely you agree that there can be circular motion without you flying into the door. That is because the car is pulling you in a circle. If the door were to come off its hinges, it would be because of the collision between the person and the door. This transfer of momentum could be calculated without involving circular motion (assuming you know the momentum that has resulted from the person being accelerated by the car), because the only objects involved are the door and the object. Relative to each other, what I drew is all that is happening. When calculating Newton's third law, you can break it down into components, and calculate it as I did.

 

I would like to make a correction though. I said there were no centripedal or centrifugal forces involved between these two. In hindsight, the normal force that the door exerts on the person is a centripedal force (in addition to the car), that is opposed under Newton's third law by a component of the force that the person exerts.

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