# Something i noticed and would like explained.

1. Sep 9, 2004

### .....

first of all, hi. im new here, im a highschool student, love my physics.

anyway, i came across this when i was doing an experiment for school...it doesnt really relate to my experiment but i found it interesting and i'd like to know why it happens.

I dropped a marble through a cylindrical container of water, of radius approx. 5cm, and as expected it fell reasonably quickly, though some resistance was noted.

I repeated this with a container of radius approx 2cm, and i noticed it took significantly longer for the marble to reach the bottom.

can anyone explain why this is to me? All i could think of was the gravitational attraction between the two masses, but i would've thought it would be too small to have such a significant effect...

if anyone can tell me why this happens i'd appreciate it.

also, what qualifications/degrees/etc do you guys have and how long have you studied for? I'm quite good in science & math compared with other students at my level, but i'm utterly baffled by the majority of the questions posted on this board just wondering what im in for if i want to be at that level some day

2. Sep 9, 2004

### HallsofIvy

No, the graviational attraction between the container and the marble (I assume those are the two masses you mean) has nothing to do with this. Try repeating the experiment without the water- you sho0uld see know difference. What you are seeing is the resistance due to the viscosity of the water. Viscosity causes water (or any other viscous fluid) to move slowly next to a solid surface. By making the cylinder smaller you are bringing the marble closer to the sides of the cylinder and the viscous effect of the sides of the cylinder affect the marble. It's harder for the water to move past the marble when it is close to the sides of the cylinder which slows the descent of the marble.

3. Sep 9, 2004

### Integral

Staff Emeritus
When the marble falls through the water it must move the water aside, in doing so some water is drug along with the marble, the closer the sides of the container are the more interaction there is with the water being drug along with the marble. So the sides of the cylinder contribute to the resistance the marble encounters in its fall. More distant sides offer less resistance thus the marble falls faster in the larger container. There is probably some critical diameter for the cylinder which anything bigger does not make any difference. Imagine what is happening to the water if you were to drop the marble through a cylinder just slightly larger then the marble, surely you can see that if the fit was very tight the marble may not fall at all or very slowly.

I have BSs in Physics and Math, you will find Physics, Math and Engineering graduate students here as well as several PhDs. The mentors have been selected by the Forum administrators and Mentor staff based on the quality of their posts and time on the board. We have recently began recognizing individuals who have shown superior knowledge with a Physics Expert ribbon, when you read a post made by a Mentor or someone with a Ribbon you can be pretty sure that you are getting the straight skinny.

Welcome to the Physics Forums.

4. Sep 9, 2004

### .....

thanks a lot!

5. Sep 9, 2004

### HallsofIvy

And we all know what BS stands for! (Ph.D is an advanced degree- it stands for "Piled higher and Deeper".)

6. Sep 9, 2004

### Integral

Staff Emeritus
Yep, thats the system, an MS is simply More of the Same.

7. Sep 9, 2004

### Artman

To further witness viscosity effecting the outcome, you could try the same experiment with a different more viscous fluid, say motor oil. The marble should drop even more slowly. Or add glycerine to the water gradually increasing the percentage and note the difference.

8. Sep 9, 2004

### pervect

Staff Emeritus
On a related point, the water has to flow from underneath the marble to above the marble. But it all has to flow through a very narrow "choke point" when the tube is almost the same diameter as the marble. This means that the required fluid velocity is very high. This high velocity increases the viscous drag. For the marble to drop a distance d in the tube of its own diameter, it has to displace it's own volume. You can compute the velocity of fluid movement at the choke point by dividing the volume of fluid by the cross sectional area that's available. This cross sectional area will be [tex]\pi (r1+r2)(r1-r2)[tex] so it's proportional to the difference in radius.