# Centripetal force ?

1. Feb 20, 2010

### cragar

so when we light 2 candles and put them on the edge of a bike tire and we spin the bike tire the candle flames lean in towards the center. And we put a glass chimney around the candles to protect the flames but the top is open . like seen here around the 4.5 minute mark in this video. . I was told that the candle flames lean in because as they go around the air inside the chimney gets pushed to the back , therefore creating a higher pressure and pushing the flame in towards the center .
But would happen if we did this in a vacuum , I mean like have a propane and oxygen injection right on the edge of the tire and a spark plug so that the flame is not obstructed by an atmosphere but it can still burn , which way would the candle flame lean , or would it lean at all .

Last edited by a moderator: Sep 25, 2014
2. Feb 21, 2010

### Char. Limit

That's not why the candle flames lean inward...

$$f=ma_c$$ applies in a vacuum.

3. Feb 22, 2010

### cragar

so your saying they lean in because the acceleration is directed in towards the middle.

4. Feb 22, 2010

### rcgldr

In real gravity or artificial gravity from centripetal acceleration, the higher density gases will move "downwards" and lower density gases, such as those in flame will move "upwards".

If you generated a flame by combining gasses in a vacuum, the the matter in the flame and the flame itself would fall "downwards" since it's denser than the vacuum.

5. Feb 23, 2010

### cragar

ok today some kid told me that the flames lean in because they move in the opposite way of acceleration , is this correct .

6. Feb 23, 2010

### willem2

NO. The direction of acceleration on a rotating wheel is pointed towards the center. Look
at Jeff Reid's post.

7. Feb 23, 2010

### Char. Limit

Correct. The acceleration is always towards the center, never outwards.

Your friend may have been thinking about the centrifugal (non)force, which seems to go outward. This force isn't a real force.

8. Feb 23, 2010

### cragar

ok i see thanks for the responses

9. Jun 8, 2010

then why do drag car tires expand outwards, that must be a centrifugal force right?

10. Jun 8, 2010

### Char. Limit

No...

That's inertia. The force is pushing the tires inward, but their own velocity is pushing them outward. Centrifugal force doesn't exist.

11. Jun 8, 2010

So your saying the mass inertia in the tire is causing them to expand outwards? I'm trying to understand this to the best of my abilities. I am thinking of a drag tire like a centrifuge, fill the tire half with sand, and when the tires go down the strip the sand will be thrown to the outside of the tire. Right?

12. Jun 8, 2010

Awww, come on... The centrifugal force does exist, despite what "overzealous teachers" say.

13. Jun 8, 2010

ah ha. thank you sir. thought I had gone crazy for a sec. however, that isn't the first time!

14. Jun 8, 2010

### Char. Limit

Well, cgaday, let's say that some large band of rubber was flying through the air at a high speed. Now, if you catch the rubber, it will stretch because of it's velocity, right? It's the same effect as the expansion of a drag tire. And if you could better explain your sand-tire thought experiment, that would be good. What do you mean by "outside"? Do yuo mean the rim of the tire, or something else?

15. Jun 8, 2010

I'm basically describing the tire as a centrifuge many construction and glass companies use to withdraw moisture from sand. Basically a large drum which rotates vertically and the sand is thrown to the outside of the drum to a barier which the sand can not penetrate but water can, and thus getting dry sand. This is called a centrifuge, so how is a tire not the same concept.

so outside of tire I mean within the actual tire, thrown against the area where the tread lies.

16. Jun 8, 2010

### vandegg

This is not a good explanation. The acceleration of a body spinning is toward the center of its rotation, but things not tied down to the object will experience the opposite acceleration due to the normal force of the surface of the spinning object acting on them. This is what is meant by "centrifugal force."

Consider the amusement park ride where people are inside of the spinning rotor, and the force pushes them up against the wall of the rotor instead of throwing them into it's center. On a roller coaster which goes around a loop, the coaster stays on the track instead of falling down.

Also i am not sure that a candle's flame has mass. If it does not then your formula would not work even in the event that the first part of my explanation was wrong.

17. Jun 9, 2010

### sganesh88

No. Centrifugal force arises when we view things in the rotating frame of the object. Normal force is a "real" one arising out of electromagnetic interactions. Centrifugal force is a pseudo.

18. Jun 9, 2010

### vandegg

No. Centrifugal force is behind the two examples i gave, with the first example, the rotor, being very similar to the original situation with the candle. You have just regurgitated two definitions without appreciating that neither one discounts what i have said.

Let me break it down for you: The rotor spins by applying a force towards its center. The tendency of the people in the rotor is to move in a straight line perpendicular to this acceleration. The wall of the ride (the normal force) pushes on the people toward the center of the ride, causing the people to push back (newton's third law).

The reason we say that this force is "pseudo" and is in "the rotating frame of the object" is because the riders in that frame feel like there is a force being generated that is pulling them against the wall, but when broken down, you get the above explanation and there is no such force. It is all the normal force combined with the inward acceleration of the ride.

This all applies to the original situation and the original explanation is probably correct. The air in the tube moves to the outside causing the candle flame to move to the inside.

19. Jun 9, 2010

### sganesh88

Why do you say there is no such force? There is a very real normal force acting on the people. And we don't call this normal force as the pseudoforce either. Pseudo centrifugal force will be observed in this rotating frame irrespective of the wall's normal force or even its existence. Imagine a rotating room with frictionless floor. A ball as seen by you (moving rigidly along with the room) should be acted on by a centrifugal force to account for its away-from-center-acceleration(making second law hold good). No normal force involved here. If we construct such a rotating room in space, there is completely no real force acting on the ball-neglecting gravitational forces from the room. Yet you observe a centrifugal force acting on the ball if you are moving along with the rotating room.

And you cant say its a force just because people "feel" a force. That's not how force is defined. Physics never really bothers to appreciate people's feelings. It has always been quite the opposite. ;)

20. Jun 9, 2010

### vandegg

The ball would stay put in your frictionless room. There would be no force acting on it and it would stay in place while the room was spinning. If you were spinning along with the room the ball would appear to be moving around the room counter to the room's actual motion.

This is exactly the point. The "centrifugal force" in this case is just a reaction to the normal force. In that same room with friction on the floor the ball would move toward the outside and then stick to the wall. If there was no wall it would move to the outside and then fly off close to tangent to the edge of the room.

21. Jun 9, 2010

### dulrich

Why does the candle flame point up? Gravity. The hot, lower density candle flame is pushed up by the colder, higher density surrounding air being pulled down.

Why does the candle flame point in? Centrifugal force. The hot, lower density candle flame is pushed in by the colder, higher density surrounding air being pulled out.

Remember that ultimately both of these forces are fundamentally inertial. That is, they come from using a non-inertial reference frame (rotating and not in free-fall). We just didn't know that about gravity until Einstein's general relativity.

Since the hotter air is less massive (for a given volume), it is more easily pushed away from the straight-line inertial motion it should travel according to Newton's First Law than the colder air.

22. Jun 9, 2010

### cragar

Also when you take a plastic jug and tie a string on a racket ball and tape it to the bottom of the jug and then fill it with water till the racket ball is submerged and floating , then put in your car and drive when you take corners the ball point towards the inside of the turn , is this the same thing as the candle flame .

23. Jun 9, 2010

### dulrich

Helium balloons in the car will do the same thing. They fly to the front and block your vision when you are trying to accelerate forward.

24. Jun 9, 2010

### sganesh88

Vandegg go through the response of Doc Al to Sophiecentaur in this thread. Posts 25 and 26. I guess you're new to PF. You can find the post number at the top right of that particular post in the format #25, #26 etc.