
#1
Mar1912, 11:27 AM

P: 96

Hey guys, I really need an insight into what exactly are pseudo forces??? When I saw the derivation, it was more of a mathematical calculation, to find the acceleration of a particle with respect to a non inertial frame of reference. But doesn't the second law of motion strictly apply to inertial frames only, so why to extend it to non inertial frames just because the expression looks like the one in the second law??? Is it meaningful to talk about forces, with the non inertial frame of reference???? And if it is not even some force, and say we are applying the law of conservation of energy wrt some noninertial frame, do we have to include it as an external force????




#2
Mar1912, 11:51 AM

Mentor
P: 40,905

(To address the title of this thread: No, I would not call pseudo forces 'real' forces. They are just an artifact of viewing things from a noninertial frame.) 



#3
Mar1912, 12:22 PM

P: 3,551

"Pseudo forces" vs. "real forces" is really a misleading naming convention, because it suggests that the "real forces" are somhow more "real", while they are also just an abstract concept and a mathematical tool. IMHO better names are:
pseudo forces = inertial forces real forces = interaction forces But you will not be able to use momentum conservation in the noninertial frames, because inertial forces do not obey Newtons 3rd Law. 



#4
Mar1912, 01:48 PM

P: 37

Pseudo forces are real forces???
I think the answer to your question depends on who you ask. A physicist will tell you that inertial forces are just a mathematical trick for being able to apply newton's 2nd law to a noninertial frame.
If you ask an engineer, he will tell you "hell yeah, of course they are real forces, just get into a fighter jet with 8 g's loopings, climbs and turns, come back to the ground and after throwing up, you'll understand why they are "real forces" 



#5
Mar1912, 03:44 PM

P: 617

To add some practicalapplication perspective, we live on one big noninertial frame (the spinning earth) that seems very inertial to our limited human senses. The introduction of pseudo forces to pretend a noninertial frame is inertial is a very intuitive approach because of where we live. Perhaps if we lived on nonspinning, nonorbiting planet, the pseudoforces approach would not seem so intuitive or so useful.




#6
Mar1912, 04:12 PM

P: 297

Why we use and introduce non inertial forces?
This is because we want to view the scenario from the point of view of a noninertial observer and use Newtons law from his frame. Using psuedo forces help us (both mathematically and physically) in describing the motion correctly. The only reason the word psuedo is used is because the origin of force seems to be unknown from that frame whereas from the inertial frame the reason of origin of force can be identified. For eg : Suppose ground is an inertial frame(we know it isn't but lets say we are performing this experiment on a different planet which doesn't rotate and is at rest) let a man moving at 2m/s^2 wrt ground (he doesnt know he is moving at 2m/s^2.infact he thinks he is in an inertial frame ) observe a car moving at 2m/s^2 wrt him (i.e the car is at rest wrt ground.the ground also appears to slide below him with acceleration 2m/s^2). he thinks some force is acting on the car but cant explain the name (or kind)of force And the source which is applying this force.(that is why psuedo forces dont obey newtons third law.As there is no identifiable source on which reaction acts) To account for all this he makes an educated guess that he is not in an inertial frame himself and is moving with some acceleration and to account for this he adds a psuedo force on the free body of diagram of car.(nothing is known about who provides this force.its just an intelligent manipulation to account for observation.) So psuedo is just used to reflect that the source of force is missing/unknown. A person at rest wrt ground will observe the car at rest abd will know that no force is acting and so will have no such problem. Now for answering your question if psuedo forces are real i would requore what A.T. said (as i feel he was spot on) Although you cannot use conservation of momentum as external psuedo force acts(same reason why you cant apply conservation of energy but will have to apply work energy theorem), you can use Newtons second law (and also third for all other nonpsuedo forces you observe like gravity) to account for new velocity's after collision. 



#7
Mar1912, 10:13 PM

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If the person inside drops the ball, then during the ball's "free fall", the person in the accelerating rocket observes what appears to be a pseudo force accelerating the ball "downwards", but from an inertial frame of reference, the ball is observed moving at constant velocity, with zero acceleration and zero real force. 



#8
Mar2012, 07:43 AM

P: 961

The most known fictitious/pseudo force called the Centrifugal force. It is fictitious since its acting alone, no action reaction. Real force obeys 3rd law, action reaction. We assume that there is a force that push us outside of the car when the car making a turn or speeding around a circular section. Same at centrifuge machine.




#9
Mar2012, 08:01 AM

Sci Advisor
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P: 26,167

it's certainly inertial (ie, proportional to mass), 



#10
Mar2312, 09:00 AM

P: 96

The main doubt I have here lies in the derivation, i guess :
We have an inertial frame F, a noninertial frame G, and a particle P, of mass m, which remains constant at all times At any time, Acceleration of G wrt F is [itex]\stackrel{\rightarrow}{a}[/itex]_{G,F} Acceleration of P wrt G is [itex]\stackrel{\rightarrow}{a}[/itex]_{P,G} Now, m[itex]\stackrel{\rightarrow}{a}[/itex]_{P,G} = m[itex]\stackrel{\rightarrow}{a}[/itex]_{P,F}  m[itex]\stackrel{\rightarrow}{a}[/itex]_{G,F} [From relative acceleration concept) Now, from Newton's Second Law applicable only to inertial reference frames, [itex]\stackrel{\rightarrow}{F}[/itex]_{P wrt F} = m[itex]\stackrel{\rightarrow}{a}[/itex]_{P,F}. Thus, m[itex]\stackrel{\rightarrow}{a}[/itex]_{P,G} = [itex]\stackrel{\rightarrow}{F}[/itex]_{P wrt F}  m[itex]\stackrel{\rightarrow}{a}[/itex]_{G,F} The problem I have is that in my book, m[itex]\stackrel{\rightarrow}{a}[/itex]_{P,G} is written as [itex]\stackrel{\rightarrow}{F}[/itex]_{P wrt G}, in a non inertial frame, whereas Newton's 2nd Law is valid only in inertial reference frames. I am able to solve kinematics problems using the relation which I think is true, without even knowing what a pseudo force is, but i still don't get the idea of the pseudo force as an external force on the particle or system of particles. 



#11
Mar2312, 09:14 AM

P: 3,551





#12
Mar2312, 09:17 AM

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P: 26,167

Hi Ashu2912!
In those frames, yes, the fictitious force is just another name for the relative acceleration (times mass) (even so, the concept of fictitious force is still helpful … for example, a horizontal "fictitious gravity" in an accelerating vehicle makes the angle of lean, or the motion of a pendulum, fairly obvious ). Most noninertial frames, however, are rotating … the concept of fictitious force (centripetal, coriolis, and euler) in those frames is very useful. 



#13
Mar2312, 09:47 AM

P: 297

Just pointing it out so that you know, we are working on a v basic case here. [itex]\stackrel{\rightarrow}{a}[/itex]_{G,F} If he has to use Newtons law in the form of F net = Ma, He will have to say that the net force acting on particle is M[itex]\stackrel{\rightarrow}{a}[/itex]_{G,F} which is mass times the acceleration he measured and also the LHS of equation. Now look at RHS. It contains two terms. One is the real force acting on particle [itex]\stackrel{\rightarrow}{F}[/itex]_{P wrt F} and some additional term m[itex]\stackrel{\rightarrow}{a}[/itex]_{G,F} which has to be introduced to make the person be able to use Netwons 2nd Law (which is using Net force= Ma from his frame .Remember he calculated a wrt his frame) This extra term is what is your fictious force. We assosciate this extra term with force because it has dimensions of force. Also a person in the accelerating frame will feel that some additional unknown forces (apart from the real ones) are acting on P The source of these forces will be unknown so he will call them as fictitous forces.  To be honest this is a vast topic with many important points and results. Download this file http://www.scribd.com/doc/8789992/Da...Solutions2004 And check out the article on fictious forces. It has the derivation for the basic case and once you read it You will have no doubts remaining :) The basic idea is that we have to modify Newtons Laws so that we can use them in non intertial frames( which is what A.T. said ) 


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