Something I have always wondered.

[KlowD9x]

Hello all, I joined this form to ask you all this question.

If you could make a funnel that has no friction on the inner surface, what would happen if you dropped an object into this funnel?

The ideal funnel: A standard 6 inch long automotive funnel.

The ideal object: A tennis ball.

I have several ideas as to what would happen, but I wanted to see what you all have to say about it.

 

[Cyrus]

It would drop into the funnel...... why do you think friction is necessary here? If anything, friction hinders the object from going through the funnel.

 

[Hootenanny]

The tennis ball would simply slide down the side of the funnel, hit the bottom and come back up again (without spinning). It would repeat this several times before coming to rest at the bottom of the funnel.

 

[Cyrus]

I beat you, I win.

 

[Hootenanny]

It would drop into the funnel...... why do you think friction is necessary here? If anything, friction hinders the object from going through the funnel.

I challenge you to pass a tennis ball through a standard automotive funnel

 

[Cyrus]

LOTS of oil.

 

[KlowD9x]

It would drop into the funnel...... why do you think friction is necessary here? If anything, friction hinders the object from going through the funnel.

The funnel is large enough at the opening to allow the tennis ball in, however, it quickly becomes smaller at the outlet, much smaller than the tennis ball.

This is why I ask what would happen if there were no friction. The tennis ball wouldn't stop, right?

 

[Cyrus]

The funnel is large enough at the opening to allow the tennis ball in, however, it quickly becomes smaller at the outlet, much smaller than the tennis ball.

This is why I ask what would happen if there were no friction. The tennis ball wouldn't stop, right?

Ok, so how exactly does a solid object pass through a hole smaller than itself...it doesn't.

 

[Hootenanny]

The tennis ball wouldn't stop, right?

Yes it would, due to the energy lost deforming the ball.

 

[KlowD9x]

Yes it would, due to the energy lost deforming the ball.

Why would the ball deform?

If there is no friction on the walls of this funnel, how it is able to push on the ball?

 

[Cyrus]

Why would the ball deform?

If there is no friction on the walls of this funnel, how it is able to push on the ball?

I think you don't understand exactly what friction is. It's a resistive force to motion. It's not a compressive force. You should read up on the fundamentals of what you are trying to describe before you use it in an argument. This is what's causing you to stumble.

 

[Hootenanny]

Why would the ball deform?

If there is no friction on the walls of this funnel, how it is able to push on the ball?[/QUOTE]
Whilst the reaction force from the funnel may not act parallel to the ball's radius, there will certainly be a radial component and therefore although the ball will not spin it will still 'bounce'.

 

[KlowD9x]

I think you don't understand exactly what friction is. It's a resistive force to motion. It's not a compressive force.

I understand that. But, I don't understand how the funnel is able to compress the ball.

 

[Cyrus]

I understand that. But, I don't understand how the funnel is able to compress the ball.

It's physically smaller than the ball. It has to compress the ball. Said differently, because the ball is falling, it will wedge itself tightly against the sides of the funnel and stop. The energy contained in the balls velocity as it falls will be converted to deforming the ball as it wedges itself upon stopping.

Really, its the ball wedging itself into the funnel, not the funnel wedging the ball into it. This can easily get into cause/effect semantics, but I think it illustrates to you the physics of what's happening and why.

 

[Cyrus]

If you wanted to, you could calculate exactly how far it could wedge itself. I just though of a simple way how to:

Assume the tennis ball is a linear spring, with some spring constant k and that the slope of the walls is dy/dx. Find the normal direction to the walls and do the dot product in the direction of gravity. This is the direction of the upwards force on the ball. The magnitude of the force is simply F=k (dx), where dx is how much the ball is physically pushed into itself because of the slope of the walls. When the dot product of the force is equal and opposite to gravity, that's how far down the ball will go into the funnel before stopping.

http://img241.imageshack.us/img241/580/tennisball.png

Notice, no where did I draw friction in this picture.

 

[junglebeast]

Surface roughness is not the only source of friction.

A funnel that has a "frictionless" surface will still cause friction if it gets too small for the ball.

Therefore the design of your experiment is a complete contradiction. It is like asking, "If you get shot in the head but the gun wasn't loaded, does the bullet kill you?"

 

[Cyrus]

Surface roughness is not the only source of friction.

A funnel that has a "frictionless" surface will still cause friction if it gets too small for the ball.

That's an improper use of the word friction in your description. There is a resistance normal force, it is not due to friction. As per the problem, it is a frictionless funnel.

Therefore the design of your experiment is a complete contradiction. It is like asking, "If you get shot in the head but the gun wasn't loaded, does the bullet kill you?"

....what......

 

[Jimmy Snyder]

There will be some energy loss associated with air resistance. So put the whole shebang in a vacuum. There might be some energy loss due to deformation as gravity pushes the ball against the funnel, so get rid of the funnel. Now you have a planet orbiting a star. The system goes for billions of years without running down.

 

[Cyrus]

There will be some energy loss associated with air resistance. So put the whole shebang in a vacuum. There might be some energy loss due to deformation as gravity pushes the ball against the funnel, so get rid of the funnel. Now you have a planet orbiting a star. The system goes for billions of years without running down.

Enter black hole.

 

[Nick89]

*Cue dramatic music*


Anyhow... To the OP: If you drop a tennisball onto a horizontal frictionless surface, would it then still pass through because there is no friction? Of course not, it would bounce just the same as if there was friction (assuming perfectly perpendicular).

If you drop a tennisball on a sloped frictionless surface, would that change things? Would the ball pass through now?

The funnel is no different, except for the fact that it has a slope at all sides.

 

[Cyrus]

*Cue dramatic music*


Anyhow... To the OP: If you drop a tennisball onto a horizontal frictionless surface, would it then still pass through because there is no friction? Of course not, it would bounce just the same as if there was friction (assuming perfectly perpendicular).

If you drop a tennisball on a sloped frictionless surface, would that change things? Would the ball pass through now?

The funnel is no different, except for the fact that it has a slope at all sides.

...what?

 

[Nick89]

...what?

 

[Cyrus]

If you shot the tennis ball fast enough to the point where it could deform to the inner diameter of the tube, then it would pass through. But that's beyond reasonable for what were talking about.

 

[KlowD9x]

*Cue dramatic music*


Anyhow... To the OP: If you drop a tennisball onto a horizontal frictionless surface, would it then still pass through because there is no friction? Of course not, it would bounce just the same as if there was friction (assuming perfectly perpendicular).

If you drop a tennisball on a sloped frictionless surface, would that change things? Would the ball pass through now?

The funnel is no different, except for the fact that it has a slope at all sides.

On a single slope? Yes, that would change everything. The ball would slide, would it not? If it were in a vacuum with no other forces (like gravity) present, the ball would continue on at the same speed till the surface ended, right?

 

[Nick89]

Cyrus, you seem to be under the impression that I am implying the ball passes through the funnel... Where did I say that? Because that's not at all what I meant to say.

KlowD9x:
No, the ball would bounce off the slope. Why would it stick to the slope? How does friction play a role in that?

EDIT
I am talking about dropping the ball onto the slope, of course. If the ball starts off on the slope and you just give it an initial velocity, then yes it would slide, without rolling or slowing down. But if you drop it from a height onto the surface, it would simply bounce off like any other slope. The only difference friction makes is that it might cause the ball to start to rotate slightly.

 

[Cyrus]

Cyrus, you seem to be under the impression that I am implying the ball passes through the funnel... Where did I say that? Because that's not at all what I meant to say.

No, no. I understand what you meant. The analogy isn't correct though because given enough speed, the ball - could- pass through the funnel.

So that it doesn't seem I am contradicting my earlier posts, the ball in consideration here is no longer a solid object. It can elastically deform, which is different from the first, simpler example I gave and is an important distinction.

 

[junglebeast]

That's an improper use of the word friction in your description. There is a resistance normal force, it is not due to friction. As per the problem, it is a frictionless funnel.

Friction is an approximation to something which can never truly be zero, because when you press two molecular structures together with enough force there will be electromagnetic interactions between the molecules of the two objects, and it will resist the direction of movement, not just in the normal direction. The normal component of this is approximated as the "normal" force, and the tangential component is approximated as the "friction" force. So by saying that there is a funnel with no friction is like saying there is a funnel that is not made out of molecules.

 

[Cyrus]

Friction is an approximation to something which can never truly be zero, because when you press two molecular structures together with enough force there will be electromagnetic interactions between the molecules of the two objects -- this is friction. So by saying that there is a funnel with no friction is like saying there is a funnel that is not made out of molecules.

Theoretically, sure. Practically, not always. Do the analysis, and run the experiment. If the result from the experiment agrees well with the analysis ignoring friction - you can ignore it.

 

[Pupil]

To the OP: the ball will stop (and compress slightly) when it reaches the bottleneck of the funnel. You may not have friction, but you still have the normal force to contend with.

 

[junglebeast]

Theoretically, sure. Practically, not always. Do the analysis, and run the experiment. If the result from the experiment agrees well with the analysis ignoring friction - you can ignore it.

I guess it depends on your definition of friction, which is really just an approximation to something real. My point is that the underlying force behind "friction" and the "normal force" (which are not real independent forces) is the same thing, and it is present here, regardless of the value of the "Coefficient of friction."