# Assorted questions

1. Jul 8, 2004

### Zorodius

I have three unrelated questions about concepts in physics. I would appreciate any answers you guys can provide

1) Does the definition of "fluids" include something like fine sand? Or numerous small pebbles?

2) Suppose you're sealed inside a large box in outer space, with whatever you'd like to bring along. Is it possible, by doing experiments inside of the box, but without looking outside of it, to determine whether the box and all of its contents are being accelerated by some external force?

3) My book describes the "Rotor", an amusement park ride where patrons enter a large mechanical cylinder. The cylinder begins to spin quickly, and the riders find themselves pinned against the walls of the cylinder. Pinned strongly enough, in fact, that once the Rotor is up to speed, the floor can drop away and the riders will not fall, through a combination of static friction and a normal force from the walls of the Rotor.

The way my book sets up the problem, the static frictional force is directed only to oppose gravity (straight up), and does not have a component tangent to the Rotor's movement. Is that right? I believe that it is not, because if that were true, the Rotor would be unable to change the rider's angular speed by spinning faster or slower.

2. Jul 8, 2004

### AKG

1) See here. Based on this, I would guess the answer is no.

3. Jul 8, 2004

### LURCH

2) it would be impossible to determine the difference between whether your box was being accelerated by some external force, or sitting still within a gravitational field. By observing the tendency of objects within the box to move toward one side of the box with a uniform motion, you could determine that one of these two things was happening, but you could not determine which one.

4. Jul 8, 2004

### Zorodius

If the entire box and every atom in it is subjected to the same external acceleration, objects would have no tendency to move toward one side or another, would they?

5. Jul 8, 2004

### arildno

The physics describing "fine sand" or small pebbles is called "granular physics", I believe (or physics of a granular medium).
From what I've heard about it, it's really nasty..

6. Jul 8, 2004

### AKG

I think so. Net Force = mass x acceleration. Apply a constant force to an object of some mass, and see if the measured acceleration matches the calculated acceleration. If not, that means that the applied force is not the net force, meaning there is some external force.

7. Jul 8, 2004

### jcsd

2) actually tidal forces would usually allow you to determine whether you you were in a graviational field or not as objects(unless we're in one of those silly gravitational field generated by flat infinite sheets) in your box nearer the source would be subject to more force.

3)As it happens I was watching a tv program about this the other day. Think about how friction usually acts, imagine applying a force to push an object along the ground and compare the direction the frictional force acts in.

8. Jul 8, 2004

### AKG

3) First of all, it might be that once it's up to speed, the angular acceleration isn't increased or decreased, and the only increase/decrease that goes on happens when the floor is in place, and thus the floor does the tangential acceleration. Secondly, spinning faster means that the centripetal force is increased. So, you don't have to increase a tangential force, but a radial force. This radial force would be the normal force, which you said was one of the ones included. I'm note entirely sure, but one of these concepts might help.

9. Jul 8, 2004

### jcsd

basically in the non-inertial frame of the person on the rotor you have the 'fictional' centrifugal force which is analogous to gravity in the way we normally consider friction in simple examples, the reaction force which is opposite and equal to the centrifugal force and analogus to the reaction force, perpendicualr to these forces you have gravity which is simlair to the force we apply to attempt to move the object, so the frictional force must act in the opposite direction to the graviational force.

10. Jul 8, 2004

### LURCH

2) it would be impossible to determine the difference between whether your box was being accelerated by some external force, or sitting still within a gravitational field. By observing the tendency of objects within the box to move toward one side of the box with a uniform motion, you could determine that one of these two things was happening, but you could not determine which one.

11. Jul 8, 2004

### Staff: Mentor

Rotor question

Once the rotor is up to speed, there is no tangential acceleration and thus no tangential component of static friction. As someone mentioned, I believe that usually the rotor is brought up to speed before the floor is dropped, so that the tangential acceleration is produced by a combination of forces from the floor and the wall.

But once the floor is gone, if the rotor starts spinning faster it must tangentially accelerate the person and this requires a tangential component of friction. The normal force of the wall will keep the person moving in a circle, but it won't change his speed.

12. Jul 8, 2004

### jcsd

As I said before this is only true in a for highly localized boxes.

13. Jul 9, 2004

### Zorodius

Thanks a lot for the replies!

Thanks, that's what I wanted to make sure I was clear on - that the centripetal force cannot change the person's angular speed, and that a tangential force is required to do this.

Regarding the box question (#2), am I correct in reading the replies to mean that, if the acceleration is uniform (not mildly stronger at one side or another of the box due to a gravitational field), then from the perspective of the person inside the box, you cannot detect the acceleration?

14. Jul 9, 2004

### LURCH

First, I am glad that the qualification has been made regarding differential from one end of the box to the other. I thought of it shortly after posting, but have not an opportunity to return and correct myself.

No, this is not really correct. The acceleration is detectable in the form of a force that tends to move objects toward one direction within the box. If there were no acceleration, this force would not be present, and things within the box to remain in their position; stationary relative to the box itself. If things tend to move toward one end of the box, then there is either a gravitational field coming from that end of the box, or the box is accelerating towards the opposite end. Which of these to it is would only be detectable if we had some means of detecting the gradient or title forces that differentiate the amount of force at one end of the box from the amount of force at the other end.

15. Jul 9, 2004

### Zorodius

If a uniform acceleration is applied to the box and every element of every object within it, then from the perspective of the person inside the box, the objects would not have such a tendency, would they?

Last edited: Jul 9, 2004
16. Jul 9, 2004

### Staff: Mentor

Right.

I would agree: if the box and everything in it are being uniformly accelerated, then you would not be able to tell.