View Full Version : need help on circular motion! plz help.
Originally posted by Bailey
For horizontal circular motion, a string tie to a ball that is rotating horizontally, i think that there is no net force toward the centre and that there will never be any.[b]
Wrong. Without the inward-directed force, the centripetal force, the ball would fly off along a straight line.
[b]Is at rotational equilibrium because is rotating at contant velocity, which also mean no net force.
Wrong. Its speed may be constant, but its velocity is not. Velocity is a vector quantity -- it has magnitude and direction. The magnitude of the velocity (speed) may be everywhere the same, but the direction quite clearly is not.
I'm not sure if I'm right or not, but i think that people got the idea of net foce toward the centre is that according to newton first law, if there is no net force acting on an obect, it will either remain at rest if it were at rest or continues moving at constant velocity in a straight line if it were moving... And the sum of the force acting to rotate the ball must point toward the centre.
Absolutely correct.
BUT!!! if i were to look at it the other way around, the ball doesn't goes toward the center, since it have the same radius everywhere while rotating
When we say the ball moves towards the center, we are saying it moves towards the center as compared to the path it would take without the centripetal force.
I personaly thinks that the tension force is cause by inertia.
Exactly. Swing a ball on a string and observe that the string is taught and you have to provide centripetal force to keep the ball moving in a circle.
And as most people knows that if you let go of the string at any given point, it will goes in a straight line(away from the center)according to newton's first law of inertia.
Absolutely correct.
And if you were to add the sum of all the inertia force, it should point away from the centre.
Absolutely. This 'inertia force' is the centrifugal (NOT to be confused with the centripetal force), which is a ficticious force. Given a suitable coordinate system, you can 'remove' the centrifugal force.
Which this sum of inertial force should cancle out with the the sum of the force that point inward toward the centre so that the ball will rotate at constant velocity and in a circle.
You're getting caught up in the difference between real and ficticious forces, like virtually all high school students. In the absence of any force, the ball would move in a straight line. Obviously, the ball is not moving in a straight line, so there must be some force. That force is directed towards the center.
i'll give an example, if you were to add some fruits on a large plate, and spin it. does the food goes toward the center or fly away from the centre?
Centripetal force is only required to make the fruits go in circular motion. When they fly off the plate, they're not moving in circular motion! If you put them in a bowl instead, and spin the bowl, you'll see that the fruits stay put, because the walls of the bowl push them towards the center.
Another example, imagine you are inside a hollow cylinder and it star rotating horizontally very fast, you would feel the pressure on your back pushing toward the center. But this should not mean that there is a net force acting toward the center.
Of course there is a net force. The wall is providing it. If the force were not present, you'd fly off in a straight line.
The reason you feel the pressure is that is a action-reaction force,which according newton's third law, that force exist because of you body exert a force away from the centre. Which therefore i conclude that there is a force acting away from the centre, which lead to a reaction force exerting in the opposite direction.
There's quite a lot of misconception in that paragraph. When you spin in the hollow cylinder, your body presses against the outer wall, in attempt to continue moving in a straight line. The wall provides an equal and opposite force, pushing you towards the center of the ride. The net result is that you move in circular motion.
Same things occur when i ask a physic teacher,which he also get mad at me for asking that question, probably because he can't even come up with any idea to prove me wrong. And all he said was tell me to go away after the conversation. As far as i recall no one have actually said that i'm wrong, except for that saying that i ask weird question.
He probably got mad at you because he continued to say the same things over and over again, and you continued to refuse to critically think about them again and again. This may, in fact, just happen again here. You've already demonstrated knowledge of what the textbook says -- you repeated it very precisely above. However, you're not thinking critically enough to understand how the textbook is entirely correct.
So am i'm wrong or right? please explain elaborately, i don't mine if any of you have different idea and say that i'm wrong(actually i'll be very gratful if anyone can prove to me that i'm wrong). Thank you for spending your time reading this( even though it have lots of grammer problems). I'm looking forward to your reply!!!
[:)]
You are wrong. If the net force on a ball on a string were outwards, the ball would break the string, swing away from you and disappear into the distance. The net force is, in fact, directed inwards, causing the ball to deviate from its "preferred" straight-line motion and move around the center of the circle.
- Warren
chroot, i'm not saying that the netforce point outward, but i'm saying that it COULD. Like the example of the ball with the string, if you were to swing it hard enough, so hard that the string actually break(since it could withstand a limited tension) then the ball should fly away from the circle in a straight line. So, this shows that the there is a net force outward away from the circle. And this is what i meant by a net force acting outward away from the circle, thats a net force could exist if you swing the string hard enought.
And you should be able to feel that "pull" from the string.
WHICH MEANS THAT I'M NOT SAYING THAT THERE IS A NET FORCE ACTING OUTWARD NOR INWARD WHILE IN HORIZONTAL CIRCULAR MOTION.
Centripetal force is only required to make the fruits go in circular motion. When they fly off the plate, they're not moving in circular motion! If you put them in a bowl instead, and spin the bowl, you'll see that the fruits stay put, because the walls of the bowl push them towards the center.
well thats a different case, since there is a horizontal force provided by the bowl acting inward toward the centre,DUE TO ITS SHAPE.
And gravity acting down on the fruit, which it push against the bowl, which then provide a normal force. Because of that normal force there is a horizontal force pushing the fruit toward the centre.
BUT, like the example with the swing, if you were somehow able to cause the bowl to spin extremly fast, the fruit should provide a greater force acting outward against the wall of the bowl. Since typical fruit are relatively fragile. But if you were to add rocks onto the bowl then spin it extremly fast, it will provide a much greater force acting on the wall of the bowl. I would think that the rocks have a possibility to overcome the reaction force of the bowl, and fly away from the centre.
o yeah, the reason the teacher got mad is b/c he can't explain it and use back exactly the same words as the book, showing a lack of understanding of the circular concept. He aggree with everythings i said, but he just can't explain it. He's like most student in my class are all using exactly the same word as the book. And whatever the book said they take in immediately. I perfer to take in things that sound reasonable and i actually understand it, i don't want to simply memorize it.
anyway, so is there a force acting outward away from the centre or not? Thats really my main question.
And thank you Chroot for your feedback, i really appreciate it.
Chi Meson
Aug6-03, 12:55 PM
quote from previous exchange:
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And as most people knows that if you let go of the string at any given point, it will goes in a straight line(away from the center)according to newton's first law of inertia.
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"Absolutely correct."
Sorry, absolutely NOT correct. IF you let go of the string at any given point, it will go in a straight line tangent to the circle. That is, in a direction 90 degrees from "away from the center." Go ahead and try it.
The reason is, the object keeps going in the same direction that it was moving at moment you let go.
Originally posted by Chi Meson
Sorry, absolutely NOT correct.
Good catch. I read rather quickly and thought that he understood that the ball would fly off along a tangent.
- Warren
i look up that "centrifugal force" that chroot was mentioning about.
so let me get this straight,there are ACTUALLY 2 FORCES acting in opposite direction for horizontal circular motion, one is the centripedal force which act toward the centre and other is the centrifugal force which act outward away from the centre?
Originally posted by Bailey
So, this shows that the there is a net force outward away from the circle.
Let's look at a hand holding a string attached to a ball moving in circular motion.
M <--- ---> T T <---
hand ------------------ ball
The ball experiences a centripetal ("toward the center") force, of magnitude T. The hand experiences a centrifugal ("away from the center") force, also of magnitude T. The string bears the tension, T.
There is only one force applied to the ball, the centripetal force. The ball is constantly changing velocity as a result of this constant centripetal force.
There must be an additional force applied to your hand by your muscles (labelled M), to counteract T and keep your hand in the same place.
well thats a different case, since there is a horizontal force provided by the bowl acting inward toward the centre,DUE TO ITS SHAPE.
It makes no difference if the centripetal force is applied by a string tied to the fruits, or by the wall of the bowl. It's the same force, and the fruits move the same way.
And gravity acting down on the fruit, which it push against the bowl, which then provide a normal force. Because of that normal force there is a horizontal force pushing the fruit toward the centre.
Neither the normal force nor gravitational force has anything to do with circular motion in a horizontal plane. Don't complicate the problem needlessly.
I would think that the rocks have a possibility to overcome the reaction force of the bowl, and fly away from the centre.
Of course -- when the bowl is no longer able to provide the necessary centripetal force to the rocks, the rocks will no longer move in circular motion. They will fly off along straight lines, tangent to the circle.
o yeah, the reason the teacher got mad is b/c he can't explain it and use back exactly the same words as the book, showing a lack of understanding of the circular concept.
Using the exact same words as the book is in fact an example of his understanding.
He aggree with everythings i said, but he just can't explain it.
Well, I agree with very little you say -- and I expect that he agrees with very little, also.
And whatever the book said they take in immediately. I perfer to take in things that sound reasonable and i actually understand it, i don't want to simply memorize it.
I'm trying to help you understand it, but you're fighting me tooth and nail. Stop your whining and THINK.
anyway, so is there a force acting outward away from the centre or not? Thats really my main question.
No. In circular motion, the only force acting on the moving body is the centripetal ("towards the center") force.
- Warren
Originally posted by Bailey
i look up that "centrifugal force" that chroot was mentioning about.
so let me get this straight,there are ACTUALLY 2 FORCES acting in opposite direction for horizontal circular motion, one is the centripedal force which act toward the centre and other is the centrifugal force which act outward away from the centre?
Not on the same body, no. On the moving body, there is only the centripetal force. On the string or wall or other device, there is a centrifugal force.
The idea that the fruits in the spinning bowl push against the wall is identical to the idea that the wall pushes against the fruits -- by Newton's third law. This is source of your confusion.
- Warren
so Chroot, if you were to draw a free body diagram, would you include the centifugal force too, alone with the centripetal force?
Originally posted by Bailey
so Chroot, if you were to draw a free body diagram, would you include the centifugal force too, alone with the centripetal force?
It depends on which body you're drawing.
If you're drawing a free-body diagram of the body moving in circular motion, there is only one force -- the centripetal.
If you're drawing a free-body diagram of the hand holding the string, there are two forces -- one centrifugal, and one provided by the muscles of the hand to counteract it.
I've already said (and drawn) all this.
- Warren
Chroot,is there centrifugal force in this example(click on the link)?
http://us.f2.yahoofs.com/users/3f316394_13e3d/bc/__hr_rotation.jpg?bcnfWM_AiTTDmCdF
Originally posted by Bailey
Chroot,is there centrifugal force in this example(click on the link)?
http://us.f2.yahoofs.com/users/3f316394_13e3d/bc/__hr_rotation.jpg?bcnfWM_AiTTDmCdF
No. Only the centripetal force, gravity, and the friction force against the wall. The answer is (B).
- Warren
Originally posted by Bailey
doesn't the person lying against the wall mean a force is pointing outward, which increase the normal force which therefore increase the force of friction acting upward?
The question asked for the forces applied to the person, NOT to the wall. There is only a centripetal force applied to the person. There is a centrifugal force applied to the wall, but the question did not ask about the wall.
- Warren
Originally posted by Bailey
then why do they show the arrow point downward, isn't that a force acting on the wall by the person?
No, that's the force of gravity on the person.
In a free-body diagram, you only show forces acting on ONE FREE BODY. In this case, the person. Forces felt by the wall are not shown.
- Warren
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