Forces on a falling sphere in a liquid

AI Thread Summary
The discussion focuses on modeling the forces acting on a sphere falling through a Newtonian fluid, specifically addressing Stokes' drag, buoyancy, and gravity. Participants clarify that the velocity in Stokes' drag formula refers to the sphere's speed, while the buoyancy formula uses the sphere's volume. A caution is noted regarding the size of the sphere relative to the gap between air molecules, suggesting the need for corrections based on Robert Millikan's Oil Drop experiment. The conversation emphasizes the importance of accurately applying these principles to ensure a correct model. Understanding these forces is crucial for effective modeling of falling spheres in fluids.
Overv
Messages
1
Reaction score
0
I want to design a model for falling spheres in a Newtonian fluid and I just want to make sure I've got everything right. Are these all of the forces that apply on an object in a liquid?

4ca08f628070e-fluid.png


I have the following formulas:

[URL]http://upload.wikimedia.org/math/7/2/2/722eecf17cc922626c36f3488ca290e9.png[/URL] (Stokes' drag)
[URL]http://upload.wikimedia.org/math/4/c/b/4cbadb68d4f9eade03797f90f99eae0a.png[/URL] (Buoyancy)
[URL]http://upload.wikimedia.org/math/e/5/1/e51111b22ad80b2b72ef52dccc810409.png[/URL] (Gravity)
 
Last edited by a moderator:
Physics news on Phys.org
Yes but be careful.
The V in the first equation is the velocity of the falling sphere, but in the second it's the volume of it.
 
Yes, don't forget that if the sphere size is comparable with the gap between the air molecules then you will have to make a correction for stokes law based on Robert Millikians Oil Drop experiment
 

Thread '1:1 versus 2:1 Mechanical Advantage debate'

The rope is tied into the person (the load of 200 pounds) and the rope goes up from the person to a fixed pulley and back down to his hands. He hauls the rope to suspend himself in the air. What is the mechanical advantage of the system? The person will indeed only have to lift half of his body weight (roughly 100 pounds) because he now lessened the load by that same amount. This APPEARS to be a 2:1 because he can hold himself with half the force, but my question is: is that mechanical...
View full post »

Thread 'average velocity as a weighted mean'

Hello everyone, Consider the problem in which a car is told to travel at 30 km/h for L kilometers and then at 60 km/h for another L kilometers. Next, you are asked to determine the average speed. My question is: although we know that the average speed in this case is the harmonic mean of the two speeds, is it also possible to state that the average speed over this 2L-kilometer stretch can be obtained as a weighted average of the two speeds? Best regards, DaTario
View full post »

Thread 'Newton's first law?'

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?
View full post »

Similar threads

Calculating the drag force on a spherical object with holes
Replies
16
Views
3K
Viscosity by Falling Sphere Equations
Replies
2
Views
1K
I Calculating Water Pool Depth: Factors to Consider in Fluid Dynamics
Replies
13
Views
2K
Stokes Law/Drag Force corrections?
Replies
1
Views
4K
Bubbles in a falling glass of champagne
Replies
20
Views
5K
How Do Viscous Interactions Differ in Vertical and Horizontal Geometries?
Replies
31
Views
3K
I Drag equation - relative flow velocity
Replies
11
Views
3K
Air drag force: Why heavy objects accelerate faster?
Replies
2
Views
2K
Does my understanding of work, velocity, and friction make sense
Replies
2
Views
2K
I Force applied on sphere by turbulent flow
Replies
28
Views
2K

Hot Threads

Recent Insights

Back
Top