Testing Drag Force on a Sphere

In summary: Sounds good. Just make sure you use a large enough cylinder and wait long enough for the ball to reach terminal velocity.
  • #1
MarchON
16
1
Hi all. This is my first question on these forums.

I am given a task to test out whether or not FD=1/2 CDAρv2 is a good model to test drag force.
Where CD is described by Reynold's Number.

We have a balloon a small ball and a myriad of basic physics lab equipment.What is a experiment I could do that would test this model and its reliability?

I'm thinking of dropping the ball into the graduated cylinder and timing it hitting the bottom to get its velocity. I have the density of water, as well as the cross sectional area (it's half the area of the ball, right?). When it comes to CD, the drag coefficient, I'm simply confused about determining linear size of the object to obtain Reynold's number. After I figure that out though, I think I'll be good.

Is this a good idea? Will it accurately test the model? My only confusion is that if I'm testing to see if it's reliable, don't I need something to compare? I have nothing else.

Thanks!
 
  • Like
Likes Greg Bernhardt
Physics news on Phys.org
  • #2
Sounds pretty good. Just make sure that the cylinder diameter is large compared to the diameter of the ball. Consider using other fluids in addition to water, so you have a range of viscosities and densities. Consider using different ball diameters and different ball densities. Make sure you wait long enough in the experiment for the fall to reach terminal velocity. Don't let the ball come too close to the bottom of the cylinder. You should do calculations in advance to make sure that you cover the wide range of Reynolds numbers present in the correlations in the literature. Try to measure the temperature of the liquid so that the viscosity is known in each test.

Chet
 
  • #3
Thank you!

Also, I think I figured out that the linear distance is the diameter of the ball, right?
 
  • #4
MarchON said:
Thank you!

Also, I think I figured out that the linear distance is the diameter of the ball, right?
Yes. Also check out web sites on Falling Ball Viscometry and Falling Ball Viscometer Corrections.

Chet
 
  • #5
MarchON said:
Thank you!

Also, I think I figured out that the linear distance is the diameter of the ball, right?

The cross-sectional area of the ball is not equal to half the surface area of the ball, but the area of a circle whose diameter is the same as that of the ball.

The cross-section, i.e. the section obtained when cutting the ball in two, is a circle.
 
  • #6
SteamKing said:
The cross-sectional area of the ball is not equal to half the surface area of the ball, but the area of a circle whose diameter is the same as that of the ball.

The cross-section, i.e. the section obtained when cutting the ball in two, is a circle.

Ohh thank, you! Saved me.
 
  • #7
MarchON said:
Ohh thank, you! Saved me.
Just to chime in: don't forget about:

1) the graduated cylinder confines the fluid through which the ball falls, altering the flow field and complicating the measurement.
2) the ball's velocity is not a constant- but a measurement of the 'terminal velocity' may suffice
3) If the object is rotating while it falls, there will be another complication you have to account for.

Otherwise, go for it!
 

Related to Testing Drag Force on a Sphere

1. What is drag force?

Drag force is a force that opposes the motion of an object through a fluid (such as air or water). It is caused by the interaction between the object and the fluid, and is dependent on the object's shape, size, and velocity.

2. How is drag force measured?

Drag force can be measured using a variety of methods, including wind tunnels, flow tanks, and computer simulations. In the case of testing drag force on a sphere, it can be measured by measuring the force exerted on the sphere as it moves through a fluid at a constant velocity.

3. Why is it important to test drag force on a sphere?

Understanding the drag force on a sphere is important in many fields, including aerodynamics, oceanography, and sports science. It can help engineers design more efficient vehicles, predict the behavior of objects in fluid environments, and improve athletic performance.

4. How do you calculate drag force on a sphere?

The drag force on a sphere can be calculated using the equation FD = 0.5 * ρ * v2 * CD * A, where ρ is the density of the fluid, v is the velocity of the sphere, CD is the drag coefficient (dependent on the shape of the sphere), and A is the cross-sectional area of the sphere.

5. How do you minimize drag force on a sphere?

To minimize drag force on a sphere, one can reduce its cross-sectional area, increase its velocity, or modify its shape to have a lower drag coefficient. For example, a streamlined shape with a pointed front and tapered back will experience less drag compared to a sphere with a larger cross-sectional area and a more rounded shape.

Similar threads

I Drag Coefficient of a 'flexible' object (e.g. a piece of paper)
    • Mechanics
Replies
4
Views
425
Calculating the drag force on a spherical object with holes
    • Mechanics
Replies
16
Views
2K
Help with drag coefficient calculated from ballistic coefficient
    • Mechanics
Replies
3
Views
4K
Relationship between Drag Force & Viscous Torque on USA Football
    • Mechanics
Replies
6
Views
744
B Terminal Velocity Equation in vertical cylinder with some fluid
    • Mechanics
Replies
2
Views
637
I Drag equation - relative flow velocity
    • Mechanics
Replies
11
Views
2K
I How is the Reynolds number derived (is my derivation wrong)?
    • Other Physics Topics
Replies
6
Views
2K
I Force applied on sphere by turbulent flow
    • Classical Physics
Replies
28
Views
810
How to estimate a constant drag coefficient of a boat?
    • General Engineering
Replies
8
Views
5K
Intuitively, which of these coroplast fins has less drag?
    • Mechanics
Replies
9
Views
2K

Hot Threads

Recent Insights

Back
Top