# Centripetal force demo dishonesty

1. Apr 8, 2014

### d4rr3n

I find physics classroom demonstrations of centripetal force dishonest, they often resort to trickery to demonstrate this apparent "inward pull" on a body in circular motion. Two examples follow

1, Centripetal force "pulling" a candle flame towards the center of a spinning turntable

2, Centripetal force "pulling" cork/bob floating in water towards the center of a spinning

In the first demonstration air is being centrifuged outwards and the displacement of air is causing the flame to point inwards, it is actually being pushed inwards not pulled inwards.

In the second demonstration its the same basic principle, water is centrifuged outwards causing the bob or cork to point inwards.

In actual fact all demonstrations to demonstrate this apparent inwards force have resorted to trickery. We know that in practice there is no inward force if there were our clothes would be projected inwards whilst spinning in a spin dryer, we know that in actual fact the opposite is true.

Is it ok to resort to misdirection and trickery to demonstrate the apparent "inward pull" on a body in circular motion?

Last edited by a moderator: Sep 25, 2014
2. Apr 8, 2014

### Staff: Mentor

This is exactly the same mechanism that causes a stationary candle flame to rise straight up.... It's being pushed upwards by the pressure of the air around it being drawn down by gravity.

You'll find a better example in the spin cycle of the washing machine, where the clothes are forced inwards by the basket while the water escapes through the holes in the basket.

3. Apr 8, 2014

### AlephZero

I agree the verbal explanations in the first two videos were poor. But the physics doesn't change because it is explained badly.

If you want to understand the candle and fishing float demos better, you can eliminate "rotation" with this experiment. Get a helium balloon and hold it on its string inside a car, so it is free to move in the air. Watch how it moves when the car accelerates and brakes in a straight line, and then how it moves when cornering at constant speed if possible. Close all the windows and air vents and turn off the aircon, to seal up the air space inside the car as much as possible.

The candle flame, fishing float, and balloon are all lighter than the surrounding air or water. That's the reason they appear to move "the wrong way" compared with "common sense".

4. Apr 8, 2014

### Staff: Mentor

Frankly, what I find dishonest is misquotes. The video doesn't say that the flame is being pulled toward the center, it says that the air is being pulled into a circle, simulating gravity, making the flame point in the direction it thinks is "up". That descriptions just fine.

The second video says an "object" being pulled toward the center, but glosses over the fact that the "object" in question is the jar full of water, not the bobber. It isn't wrong per se (the sentence sounded straight out of a textbook), it is just not fully explained.

5. Apr 9, 2014

### d4rr3n

Nothing is in fact being "pulled" towards the center, there is no real pulling force. When a candle burns in normal conditions the reason the flame points upwards is because the hot vapor of the candle is less dense then the surrounding air thus it rises. In the case of the candle put into the centrifuge the air in the bell jar is being compressed (forced to the outside). The air being more dense at the outside the hot vapor expands in the opposite direction. Nothing is being "pulled" inwards, there is no inward pull.

The spin cycle of a washing machine again nothing is pulled inwards towards the center of the drum, both the clothes and water is centrifuged to the outside of the drum.

The only time there is an illusion of the clothes being pulled into the center of a drum is in a tumble dryer when the drum changes direction of rotation however that is due to other factors and again there is no real pull towards the center of the drum.

6. Apr 9, 2014

### Bandersnatch

Only in the non-inertial frame of reference rotating with the drum.
In an inertial frame, there's no centrifugal force, and clothes are most definitely accelerated towards the centre of the drum by the drum walls, as otherwise they'd travel in straight lines as per the Newton's 1st law of motion.

7. Apr 9, 2014

### d4rr3n

What would be the resultant of the combined vectors, the tangential and the supposed centripetal force added, it would be an inwards spiral and we most definitely do not observe this in a spin cycle of a washing machine.

If we now change the vectors, one tangential as before and the other pointing outwards away from the center of the drum (centrifugal) we have a resultant force which is a spiral outwards ie compressing the clothes against the side of the drum, this fits in reality closer to what we observe.

8. Apr 9, 2014

### Staff: Mentor

There is tension in the devices pictured: they most certainly are pulling on the "objects" to keep them in "orbit" around the center.
The drum is arranged so that it pushes the clothes toward the center, but in its structure underneath is tension, pulling the parts of the drum toward the center.

9. Apr 9, 2014

### d4rr3n

The tension is only in the turntable/rotor between pivot and outer extremity of the mass, tension is caused by an outward force, were it an inward force it would be in a state of compression which again we do not observe

You mean the drum wall is slightly conical? in any case still there is no centripetal inwards force. Look at the last video of the spinning fish tank and state where this inward force is observed.

10. Apr 9, 2014

### Staff: Mentor

The wall of the tank exerts a force on the water. In which direction is this force acting?

11. Apr 9, 2014

### Bandersnatch

There is no tangential force. The only tangential vector is that of the tangential velocity, which the centripetal acceleration(via the force) is constantly changing(direction only) to keep the clothes moving in circles. There's nothing to add.

From your description, i.e., the drum is moving rather than stationary, I gather you use an inertial reference frame. In such a frame there is no centrifugal force.

12. Apr 9, 2014

### Staff: Mentor

You are dodging your own statement in the OP, not just what I said. You said "inward pull" in the OP and I said "pulling" in my post. The tension means the center is pulling the object toward it and the object is pulling back. Draw a free body diagram if you don't believe it.

No one has said that the object is pushing toward the center. That doesn't even make any sense.
No, I mean the base of the drum is a disk just like the setup in the first video. It is that disk that is in tension.
The base of the fish tank is in tension (edit: so are the sides).

Part of your issue here is that you don't seem to be grasping that all forces come in pairs. The center axis pulls the object towards it (inward) and the object pulls away from the center (outwards).

Last edited: Apr 9, 2014
13. Apr 9, 2014

### d4rr3n

The walls of the tank "resist" an outward force, if you spin the tank fast enough and had a lid the tank walls would explode. There is no inward push that I can see, the system is in tension not compression.

14. Apr 9, 2014

### Staff: Mentor

It doesn't rise just because it is less dense. In zero gravity burning products just stay there, and extinguish the fire.

15. Apr 9, 2014

### ZapperZ

Staff Emeritus
Either you are truly not seeing this, or you are purposely being difficult and refusing to learn.

Here's a challenge for you. We already have well-known solutions to the central force problem. They are written in books, they are part of students' homework assignment, etc.

Now, if you disagree with this, then please show me the solution for an outward force problem, and derive the same equation of motion that we arrive at for the central force problem. After all, the central force problem has shown ALL the equation of motion that fits very well with our observations. So go ahead, show me that you can actually get the same trajectory and motion of your system with this outward force alone.

Otherwise, this scenario of yours remains nothing more than a misguided handwaving argument.

Zz.

16. Apr 9, 2014

### d4rr3n

By tangential I am talking about the inertia of the spinning objects that would want to move in a straight line. add that to the inward centripetal and you should have an inward spiral but we don't see this.

Instead lets consider a satellite orbiting the Earth Vs a wagon wheel spinning at high speed.

In the case of the satellite orbiting the earth we have the inward pull of gravity and the tangential inertia of the satellite curving around the Earth, add those vectors and the resultant is an inward spiral so that eventually the satellite crashes to earth (lets not get into escape velocity here).

In the case of the spinning wagon wheel are the spokes of the wheel in a state of compression or tension? If they are in a state of compression we can talk about an inward centripetal force but if (and they are) in a state of tension it makes no sense to talk about inward force, tension could only be the result of an outward force (centrifugal).

17. Apr 9, 2014

### Staff: Mentor

Remember Newton's third law, the one about equal and opposite forces; the "resisting" to which you refer is the centripetal force.

The water directly adjacent to the edge of the tank is in compression from the force the tank wall is exerting on it. The tank wall is in tension from the equal and opposite force the water is exerting on the wall.

18. Apr 9, 2014

### d4rr3n

I truly do not see the inward force, if I did this discussion would not exist. In the case of the fish tank I see the base of the tank in a state of tension (the result of an outward force) and I see the two walls in a state of compression (resisting the pressure exerted by the centrifuged water). I cannot see any inward force

19. Apr 9, 2014

### ZapperZ

Staff Emeritus
The problem here is that in many of your examples, you are picturing yourself in that rotating frame! You keep talking about the "outward force", which really isn't there when you are not in that frame. It is easy to make that mistake because we all have been in a car when it curves around the corner and "feel" the outward force. But we forget that we want to go in a straight line, and the side of the car and the friction between the seat and our butts provide the inward force that pushes us in.

I suggest you look again at each of your example without trying to picture yourself in the rotating system. Look at what is pulling the objects inwards. It is there! I guarantee it! Your intuition does not match, even remotely, to the mathematical derivation of the equation of motion for a circular motion. That should mean something to you!

Zz.

20. Apr 9, 2014

### d4rr3n

If we spin the tank and the water mounded up in the center of the tank I would agree it makes sense to talk about an inward centripetal force. However the opposite is true so it makes more sense to talk about an outward centrifugal force pushing the water out from the center.

Which force is more primary, the inward resistive force offered by the walls of the tank you speak of is a reactive force ie a reaction

the outward force is more primary in this situation