Newtonian Physics tabletop demo

[DaveC426913]

I wish your help in creating a tabletop Newtonian physics demo.

My friends do not believe that the centrifugal force is imaginary. They believe that, if you swing a pail of water in a circle over your head and let the rope go, the pail will sail *away* from you (as opposed to tangential to the circle).

(see diagram)

I am sure that the point of ambiguity is that path A is perceived as if it is really path B, that the confusion lies in not knowing exactly where the snip happens. It is key to understanding that the rope and the path of the bucket are exactly perpendicular to each other a thte point of snippage. This would demonstrate that there is no outward force acting on the bucket - only inertia keeps it moving.


I wish to demonstate this in a simple and elegant way.

It needs to have
- frictionless rotation
- a way of cutting the centripetal force bearer (i.e. the thread) while clearly not affecting the path or imparting any other forces
- precision, inasmuch as I can show the point of snip, and the precise resultant trajectory

I saw this in a high school film, where they used a (frictionless, dry ice-powered, air hockey puck-like) device tied to a post by a thread, and they used a candle to cut the thread. I do not have this equipment.

My next thought was a bicycle wheel set on its axle, with a marble in a box, and a trigger. This would work, but it's large, heavy and dirty.


Can anyone think of a simple, elegant tabletop mechanism I can setup to demonstrate this?

 

[russ_watters]

On the end of the swing-arm, mount a flat plate just above the level of the table. Put a small dimple in it. Put a marble in the dimple. When the centrifugal force gets too high (I hate it when people say its imaginary...), the marble will roll out of the dimple.

If you want the marble to roll out at a specific point, put a groove in the plate and have something sticking up from the table (like a nail). When the nail goes through the groove, it'll pop the marble out of its little dimple.

 

[dextercioby]

(...)
My friends do not believe that the centrifugal force is imaginary.(...)

And they have damn good reasons for it...

Daniel.

 

[DaveC426913]

On the end of the swing-arm

Yeah - what swing arm?

Got some ideas about building a simple low-friction rotating device? A lazy susan just isn't frictionless enough and a bicycle wheel is too cumbersome.

But yeah, the idea of knocking the ball out should do, so long as they're convinced it's not imparting or cancelling any forces. (They hate to be wrong, and they can get desparate.)

 

[Doc Al]

How about the old tennis ball on a string? Have someone spin it around and let it go when it lines up with some target. Not exactly table top, but easy to do. Everyone can take turns until they are convinced that the ball does not travel to the target. (Have the doubters stand tangential to the circle.)

 

[FredGarvin]

What about having an arm that is attached to a torsion spring. On the end of the arm is a basket of some kind to hold a marble. Somewhere in the path of the arm, you place a removeable stop.

- Wind up the arm and hold in place.
- Put the stop in place.
- Let go of arm.
- Arm hits stop, ball goes flying.

 

[jdavel]

Doc Al,

That would work if "the doubters" thought the ball would fly off radially. Just set up two targets outside the circle of rotation, spaced at 90 degrees. He tries to hit theirs, and they try to his (using each other's iPods for targets and a string with a big rock would be exciting!).

But according to Dave's diagram, it looks like they say the mass flies off somewhere in between radially and tangentially. Assuming some error, that's usually what it will do, so Dave would be at a big disadvantage.

 

[Doc Al]

But according to Dave's diagram, it looks like they say the mass flies off somewhere in between radially and tangentially.

Good point.

 

[heaven eye]

first i would say centrifugal cause of inertia which resisting the change in velcocity it's the same when you in a car then you slowed down surprisly your body will still moving forward , so the centrifugal acceleration is illusory cause of inertia


heaven eye

 

[Werg22]

Just tell them that if the centrifugal force exists, why does the objects do a revolution a round one point? If there was an external force, the object would certainly not follow this motion. Just take a needle and do the string thing. It will almost not budge, since it has a very little inirtia...

 

[heaven eye]

Just tell them that if the centrifugal force exists, why does the objects do a revolution a round one point? If there was an external force, the object would certainly not follow this motion. Just take a needle and do the string thing. It will almost not budge, since it has a very little inirtia...

thats because it has a centural force(pulling force of your hand, gravitation force) is in the opposite of centrifugal (acctually its (i don't know if it was even a force) resist for the velocity which accuring in the circular motion) , whatever if you took string and tide it with a stone then taught it from other side and turned it around you it will rolling around and there is two proofs that there is a centrifugal exist :-

1) you feel that the stone want to skip from your hand and each time turning it faster then you have to exert more force (touch it strongly).

2) if that's string made of spring it will be longer each time you roll it faster.


i want say also that's circular motion has two acceleration in the same time:-
1- at = the tangent acceleration if you multiply it with its mass it will has force ( centrifugal) . (thats explain why you feel the stone trying to skip out from you).

2- ac = centural acceleration and its vector to the center of a circle ( to you !) (thats explain why moon doesn't skip from the earth


also i want to say (ac) (in acircular motion) causing of moving objects (with vector and that's another story) and (at) is why moon doesn't skip from the Earth


well you might ask two questions:-
1) you said (ac)is making moon doesn't skip from the sun what about the Earth >>>>>>>>answer is on the Earth objects are not segregated of other effects (such asd Earth gravitaion (g)).

2) so what about the moon ( you mean that's there is a gravity in the space??>>>>>>>> well yes and there is (moon distance is not so long) and there is two forces as i think so:-

fc=v^2/r
and
f=(G mmoon mearth )/r^2
(r is the radius from the Earth core to the moon core (or to the moon surface actually i don't remember)





finally i would say for your last question saying that's it has a little inertia >>>>>> well little inertia if the object was small such as stone but if it was a huge inertia such as moon inertia do you think that is it a small inertia (i let this question for you )


thank you be cause you asked me and know that iam trying my best and iam 16 years old !

my regards
heaven eye

 

[Werg22]

thats because it has a centural force(pulling force of your hand, gravitation force) is in the opposite of centrifugal (acctually its (i don't know if it was even a force) resist for the velocity which accuring in the circular motion) , whatever if you took string and tide it with a stone then taught it from other side and turned it around you it will rolling around and there is two proofs that there is a centrifugal exist :-

1) you feel that the stone want to skip from your hand and each time turning it faster then you have to exert more force (touch it strongly).

2) if that's string made of spring it will be longer each time you roll it faster.


i want say also that's circular motion has two acceleration in the same time:-
1- at = the tangent acceleration if you multiply it with its mass it will has force ( centrifugal) . (thats explain why you feel the stone trying to skip out from you).

2- ac = centural acceleration and its vector to the center of a circle ( to you !) (thats explain why moon doesn't skip from the earth


also i want to say (ac) (in acircular motion) causing of moving objects (with vector and that's another story) and (at) is why moon doesn't skip from the Earth


well you might ask two questions:-
1) you said (ac)is making moon doesn't skip from the sun what about the Earth >>>>>>>>answer is on the Earth objects are not segregated of other effects (such asd Earth gravitaion (g)).

2) so what about the moon ( you mean that's there is a gravity in the space??>>>>>>>> well yes and there is (moon distance is not so long) and there is two forces as i think so:-

fc=v^2/r
and
f=(G mmoon mearth )/r^2
(r is the radius from the Earth core to the moon core (or to the moon surface actually i don't remember)





finally i would say for your last question saying that's it has a little inertia >>>>>> well little inertia if the object was small such as stone but if it was a huge inertia such as moon inertia do you think that is it a small inertia (i let this question for you )


thank you be cause you asked me and know that iam trying my best and iam 16 years old !

my regards
heaven eye

1) Wrong. The reason why you need to exerct more force is only due to a combination of gravity and air resistance. While air resistance slows down the object horizontally, gravity drains it downward. That is why when you are turning a string you can't do it without your hand diagonally, relatively to the angle of rotation, moving it (thus a vertical and a horizontal component).

2) Wrong again. The moon does distance itself from Earth constantly. And no the tangential vector is a constant speed. You see, acceleration in a circular motion does not mean that the speed changes, but only the direction. If the vector itself alone would move (it's impossible but let say the object is stationary and the vector moves around it), in this case the speed would change. But since the speed is always the tangential to the object, its magnitude does not change.

 

[DaveC426913]

Thanks for the "just tell them" solutions and the "follow this logic" solutions, but there's no point. They need to see it. Unequivocally.

Any demo where I'm just letting something go at the right point is not going to work. It's way too imprecise. There's no way to show
- that, in the act of releasing it, there was no extra force imparted, or
- that it was released at the right point. This is the most likely cause for contention. (See attached updated diagram.) They can easily insist that it followed path B instead of path A.



Note also that a tennis ball on a string will not demonstrate this well at all. To keep the tennis ball at speed, you cannot remain still, with the tennis ball rotating about your hand, you must drag the rotational point *ahead* of the tennis ball to keep it going. That's easily enough imparted force to destroy the experiment.

 

[Werg22]

Ok I just tought of an experiment that might prove the inexistence of the force. Rotate an object on a string with a little velocity. In the first cycle notice the radius of a certain point. Do the same with the second cycle as well as for the third. You shoud see that the interval is constant. Do the same with a greater initial speed. If you get the same position difference, the centrifugal force does not exist as it would have displaced the object further since it would have had a greater magnitude. This will prove that it is a matter of inirtia and not of force.

 

[heaven eye]

thank you very much werg22 for discussion and sorry all if my info was all wrong

werg can i ask you question what about at (tangent acceleration ) does it change its direction constantly such as velocity

thank you ... and thank you all again




my regards
heaven eye

 

[Werg22]

Relativly to the object, no it always be the same force that directs it inward. Relativly to the center, yes, it moves the same as the tangent speed and the object.

 

[Werg22]

DaveC426913, have you considered the experience I proposed? If you have the technical requirement the experience will surly work...

 

[heaven eye]

Relativly to the object, no it always be the same force that directs it inward. Relativly to the center, yes, it moves the same as the tangent speed and the object.

how is it inward and tangent in the same time!

thank you again mr.werg22

 

[Werg22]

There is an acceleration toward the center, and a tangent vector. There is no tangent acceleration.

 

[SVG84R]

during a circular motion,the speed may be constant,but the velocity is constantly changing.the vector sum of the velocity points towards the center hence the acceleration is also towars the center.hope that helps.

 

[heaven eye]

during a circular motion,the speed may be constant,but the velocity is constantly changing.the vector sum of the velocity points towards the center hence the acceleration is also towars the center.hope that helps.

thank you mr. svg84r




mr.werg 22 Look mr.werg22 I know that there are two kinds of accelerations in constant speed

Tangent acceleration
central acceleration

 

[Doc Al]

I know that there are two kinds of accelerations in constant speed

Tangent acceleration
central acceleration

For circular motion at constant speed the acceleration is centripetal. There is no tangential component of acceleration.

 

[DaveC426913]

Ok I just tought of an experiment that might prove the inexistence of the force. Rotate an object on a string with a little velocity. In the first cycle notice the radius of a certain point. Do the same with the second cycle as well as for the third. You shoud see that the interval is constant. Do the same with a greater initial speed. If you get the same position difference, the centrifugal force does not exist as it would have displaced the object further since it would have had a greater magnitude. This will prove that it is a matter of inirtia and not of force.

I don't follow what this demonstrates.

A 6" string will give me a 6" radius circle, regardless of speed and force. The radius will be constant no matter how fast I rotate the object on the string because the string will hold it there.

 

[Werg22]

No the radius will increase. Of course the maximum radius would be the lengths of the string, but if you start turning at a certain speed while not stretching completely the string the radius will increase gradually. This is due to inirtia. Just to prove it, try to turn around one point at a constant speed yourself. You will notice that your original radius was increased at each complete turn. Same concept with the string. Now repeat the same experiment with a different speed and thus radius. If in both case you measure the difference of the radius the same after one complete turn, it will definitely prove the inexistence of centrifugal force

 

[Doc Al]

No the radius will increase. Of course the maximum radius would be the lengths of the string, but if you start turning at a certain speed while not stretching completely the string the radius will increase gradually.

If you are using the string to move the object in a circle, then the string will be taut. Ignoring any slight stretching of the string, the path radius will always equal the length of the string (as DaveC426913 said).

 

[DaveC426913]

No the radius will increase. Of course the maximum radius would be the lengths of the string, but if you start turning at a certain speed while not stretching completely the string the radius will increase gradually. This is due to inirtia. Just to prove it, try to turn around one point at a constant speed yourself. You will notice that your original radius was increased at each complete turn. Same concept with the string. Now repeat the same experiment with a different speed and thus radius. If in both case you measure the difference of the radius the same after one complete turn, it will definitely prove the inexistence of centrifugal force

I'm afraid I just don't follow.

 

[Werg22]

If you are using the string to move the object in a circle, then the string will be taut. Ignoring any slight stretching of the string, the path radius will always equal the length of the string (as DaveC426913 said).

I don't mean starting at the maximum radius. I mean rotate the string around a little radius and the same speed and the radius will increase, and finally reach maximum radius. Only thing, you have to make sure the two motions you will compare will have the same initial period (since the radius will increase, the period will increase. You could use two different strings, but then again the period has to be the same. We will have to try this experiment once to see the result. And yes, for stretching, it is the same since the centripetal force does not change. It stretches further because the mass gets farther and farther due to its inirtia.

Anyway, Dave, if you still septical, try this instead: take bucket filled with water and spin it on itself. When you stop it, you will see that the waves in the water are directed to the center. If centrifugal force existed, the waves would have been directed away from the center.

 

[gerben]

How about just having something like a ruler with a hole in one end over a pin so that you can rotate it, and then you put a small disk like a piece from a game of checkers on the outside. You start to rotate the ruler slowly and once it is rotating fast enough you suddenly stop it, the disk will slide a small distance over the table and you can draw a line from the disk to the point where it originally sat on the ruler.

 

[Werg22]

No the disc will move because the ruler transmited its energy to table and thus some to the disc. In any case, I have no doubt my first proposal would work. When I mean spind the string, it has to be spin first diagonally to the floor (so the radius will be the x component). You WILL see that the radius will start increasing (x compenent will increase). It's very simple. Now at each cycle measure the difference between the current radius and the previous radius. It should be constant. Now you have to repeat the same experiment, but with different speed and initial radius. Most importantly, when you repeat the experiment make sure that the initial period is the SAME as the previous one. Per example:

First experiment initial radius period equation:

T=2rPI/v

In the second experiment, since the period would be the same, the speed and the radius would change dependently. So if you choose to start with a radius twice as big as before:

T=4rPI/2v

Makes sense now?

 

[Werg22]

Now I hope you understand. I made a picture so you can see what i mean visually:

http://www.xtendspin.ca/autres/explanation.jpg

You see the string stays the same, but the radius increases at each cycle due to inirtia. When repeating same thing but with a different initial radius and speed, but same period, you will see that A1=A2, B1=B2 and so on, of course ignoring air friction.

 

[DaveC426913]

Now I hope you understand. I made a picture so you can see what i mean visually:

http://www.xtendspin.ca/autres/explanation.jpg

You see the string stays the same, but the radius increases at each cycle due to inirtia. When repeating same thing but with a different initial radius and speed, but same period, you will see that A1=A2, B1=B2 and so on, of course ignoring air friction.

While I suspect that your experiment will demonstrate the phenomenon, it does not do so in a way that is so concise and clear as to wipe away any doubts in the minds of non-believers who are not scientifically minded.

The ruler experiment OTOH demonstrates it in the simplest possible way.

 

[DaveC426913]

How about just having something like a ruler with a hole in one end over a pin so that you can rotate it, and then you put a small disk like a piece from a game of checkers on the outside. You start to rotate the ruler slowly and once it is rotating fast enough you suddenly stop it, the disk will slide a small distance over the table and you can draw a line from the disk to the point where it originally sat on the ruler.

Yah. you know what? You're right. I'm thinking too big, with my bicycle wheel.

The ruler idea will work, with a few mods
- I can't have the ruler stop, because they'll naturally claim that, by stopping the ruler, I've eliminated the force. What I have to do is knock the poker chip off the rotating ruler.
- I have to make sure I eliminate any user involvment in the demo (i.e. I can't be manually spinning the ruler, it must spin freely.) or the demonstration of - what forces, how strong they are, and when they are acting - will be muddied.

 

[Werg22]

While I suspect that your experiment will demonstrate the phenomenon, it does not do so in a way that is so concise and clear as to wipe away any doubts in the minds of non-believers who are not scientifically minded.

The ruler experiment OTOH demonstrates it in the simplest possible way.

I admit that my experiment is too complicated to produce. But it can't be denied, it is a mathematical argument. The bucket experiment is the simpler that comes to my mind now...