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Centrifugal force?

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gordonjudd
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Centrifugal force?

#51

Post by gordonjudd »

It doesn’t matter. What you see now is centrifugal force, a real force.
Aitor,
Your Mr. Knowledge needs to study this site What is a Pseudo force? and understand what he is trying to get across in his introduction.
Today we will ask a few questions such as

What is a pseudo force?
What is a fictitious force?
Are pseudo forces real?
Is gravity a pseudo force?

Alas, we will not answer these questions!

As far as I can tell, nobody knows, and nobody cares. Or at least nobody should care. If these questions have answers at all, each answer is a matter of opinion, and opinion is divided. There is no consensus. When I asked several hundred physics teachers via the phys-l mailing list, nobody seemed to have any firm opinions. I don’t have a firm opinion, either.

There is no experiment you can do that will tell you the answers to these questions.

For example, as far as I can tell, gravity can be classified as a “pseudo force” – or not. It’s six of one and half-a-dozen of the other. You can choose to do it either way, and the choice has no observable consequences.

The fact that there is no easy answer suggests that we are asking the wrong questions, and that we should ask more physically-meaningful questions instead.
We are dealing with acceleration terms not force terms in the rotating field. The pendulum does not swing to the right because there is some mysterious "real force" acting on it. In the same context a stone does not fall when it is dropped because there is a real force pushing it down. That is why he says:
For example, as far as I can tell, gravity can be classified as a “pseudo force” – or not. It’s six of one and half-a-dozen of the other.
How do we compute what experience has told us is the force we can expect for gravitational acceleration on earth? We use Newton's second law and come up with F=mg where g is the gravitational acceleration you get on the surface of the earth.

How do we compute the equivalent centrifugal force (call it real, pseudo, fictitious or what ever you want. It is 6 of one a half dozen of the other just like gravity) we use the same second law and say F=m*(centrifugal acceleration) where the centrifugal acceleration term is equal to omega.^2 (the square of the angular velocity in radians/sec) times the distance (lets call it r) you are away from the axis of rotation of the rotating field. Thus f_centrifugal=m*omega.^2*r.

Work out the numbers and you will find that f_centrifugal is the same magnitude as f_centripetal which is m*v.^2/r when you are going around a circular path with a tangential velocity (v) having a radius (r0. By convention they point in different directions; one out, one in.

Gordy
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Centrifugal force?

#52

Post by Unregistered »

Yes Gordy. And how do you compute the "force" (call it "pseudo", "fictitious" or just "inertia", whatever suits your taste) of the pendulum swinging forward?
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gordonjudd
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Centrifugal force?

#53

Post by gordonjudd »

Yes Gordy. And how do you compute the "force" (call it "pseudo", "fictitious" or just "inertia", whatever suits your taste) of the pendulum swinging forward?
Aitor,
There is no centrifugal acceleration term in that case because omega is zero.

Rather the force that keeps the pendulum from smashing into the windshield is the tension force from the string. That would be a real double ended force where the negative x directed force would be f_x=T*sin(theta) where T and theta are the varying tensions and angles you get as the pendulum swings forward as the car is decelerating.

I will leave it to you to figure out what T(t) and theta(t) might happen to be so you can work out the terms involved in for the ODE needed to solve your problem.

Gordy
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Walter
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Centrifugal force?

#54

Post by Walter »

Aitor wrote:... You must consider the first case as caused by a reactive force as well.
Absolutely. The pendulum does not appear to lean to one direction unless there is an application of force somewhere.

The pendulum example is actually more complicated than it seems because the net force acting on the pendulum bob is dependent on the angle of the string attaching it to the ceiling of the car. Let's look at a simpler system such as a cup of coffee sitting on the dash of the car. If I stop the car the cup flies forward because there is no force acting on it so its inertia keeps it moving forward as the car comes to a stop. If I am sitting stationary and step on the gas going in reverse the cup appears to do exactly the same thing - fly forward. Again, this is due to inertia because the cup is stationary and is resisting a change in its momentum. If I'm driving along at constant velocity and then turn sharply to the right the cup appears to fly to the left - again, this is inertia because the cup continued on at constant velocity while the car moved under it. If I tie the cup to the dash with a length of string and then change velocity the cup will initially fly in the direction opposite the applied force (inertia) and when it reaches the end of the string it will then move with the car (applied force).
"There can be only one." - The Highlander. :pirate:

PS. I have a flying tank. Your argument is irrelevant.

PSS. How to generate a climbing loop through control of the casting stroke is left as a (considerable) exercise to the reader.
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Centrifugal force?

#55

Post by Unregistered »

Walter wrote:
Aitor wrote:... You must consider the first case as caused by a reactive force as well.
Absolutely. The pendulum does not appear to lean to one direction unless there is an application of force somewhere.
So it seems that centrifugal force is just a special case of inertia. Which is my point from the beginning.
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Centrifugal force?

#56

Post by Unregistered »

gordonjudd wrote:
Yes Gordy. And how do you compute the "force" (call it "pseudo", "fictitious" or just "inertia", whatever suits your taste) of the pendulum swinging forward?
Aitor,
There is no centrifugal acceleration term in that case because omega is zero.

Rather the force that keeps the pendulum from smashing into the windshield is the tension force from the string. That would be a real double ended force where the negative x directed force would be f_x=T*sin(theta) where T and theta are the varying tensions and angles you get as the pendulum swings forward as the car is decelerating.

I will leave it to you to figure out what T(t) and theta(t) might happen to be so you can work out the terms involved in for the ODE needed to solve your problem.

Gordy
Fortunately this isn't the technical department so we can spare those calculations.

But if it happens that a fly is hovering around the pendulum and when the car turns left the pendulum hits the fly, the insect is going to feel an impact (ouch!). It has felt a real force against it.

If the same fly is hovering around the pendulum, and happens to be positioned in the trajectory of the pendulum when the car stops at the traffic light, the insect is going to feel the same impact (providing that the change of acceleration is the same in both cases). The insect has felt the same real force against itself.
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Marc Fauvet
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Centrifugal force?

#57

Post by Marc Fauvet »

Aitor wrote:
Marc Fauvet wrote:oh my goodness ! with all your help Aitor i've gone from being a layman simpleton to a Mr. Knowledge !!! :D :yeahhh: :D
Great! So now you can explain to me the difference between the two cases because I can't see it. :yeahhh:
well, maybe i'll take my comment back because the more i read and try to understand all this the more confusing it gets ! :whistle: :D

http://www.bronnikovmethod.com/theory.php
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Mike Heritage
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Centrifugal force?

#58

Post by Mike Heritage »

well, maybe i'll take my comment back because the more i read and try to understand all this the more confusing it gets !
Thank God for that, I thought it was just me :upside:. Interesting though. So, are the forces in a loop pseudo or real or both or neither.

Mike
It's fly casting Jim, but not as we Know it.
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Centrifugal force?

#59

Post by Unregistered »

IMHO opinion there is a very real force, Mike:
It is applied by you via the rod tip and it makes the loop turn around.
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Walter
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Centrifugal force?

#60

Post by Walter »

Aitor wrote:
Walter wrote:
Aitor wrote:... You must consider the first case as caused by a reactive force as well.
Absolutely. The pendulum does not appear to lean to one direction unless there is an application of force somewhere.
So it seems that centrifugal force is just a special case of inertia. Which is my point from the beginning.
Inertia is only about objects that are either stationary or moving in a straight line at a constant speed. Whether the car comes to a stop or veers to the side the cup keeps moving in a straight line. An object does not move in a circle because there is a net sideways force on it. An object moves in a circle when there is a net force that is always at right angles to its current direction of motion, resulting in a force which is constantly changing direction such that it is directed at a single point in space. This is by definition a centripetal force.

The cup that is sitting on my dash when I circle to the left will appear to fly off to the right to a person sitting in the car but it is actually continuing in a straight line until it contacts the car. At that point the car exerts a centripetal force on the cup and it too begins moving in a circle. There is no centrifugal force until there is a centripetal force because the centrifugal force is the equal and opposite version of the centripetal force.
"There can be only one." - The Highlander. :pirate:

PS. I have a flying tank. Your argument is irrelevant.

PSS. How to generate a climbing loop through control of the casting stroke is left as a (considerable) exercise to the reader.
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