# B Thermal velocity of a soccer ball

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1. Jan 30, 2017

### Markus Lervik

Hello,

I am an IB HL Physics student. I am thinking of doing my IA on the thermal velocity of a soccer ball. I will be using a soccer ball shooter (machine) to simulate the thermal velocity. I have a couple of questions: what is the thermal velocity equation, which would fit for a soccer ball's motion? Is there any other good way of approach to find the thermal velocity?

Thank you.

2. Jan 30, 2017

### BvU

Hello Markus,

I take it you have googled all the relevant terms, so I am a bit surprised your questions are so unspecific. Perhaps you want to study this a little and think about what it is you could simulate with your ball cannon (which, of course, is fun to play with ). You want to realize it will be rather impossible to simulate a gas of soccer balls.

Wasn't the thermal velocity something like ${1\over 2} mv^2 = kT$ , so about 12 $\mu$m/s for a 300 gram soccer ball ? You don't need a cannon for that

3. Jan 30, 2017

### A.T.

Could he mean terminal velocity?

4. Jan 30, 2017

### Markus Lervik

Sorry about the unspecific details, I meant terminal velocity. I wrote this just after having a class about thermal energy. Terminal velocity is when drag force is the same as force of gravity. This will then push the ball downwards, when the object is in mid-air. I would need to hit the ball so that the drag force equals the force of gravity. I can hit the ball with my foot, but I'm struggling to see how this could give me accurate numbers. Therefore, using the machine would help me.
Thank you.

Is there any other way that I can approach the problem.

Problem: Solving the terminal velocity of the ball.

5. Jan 30, 2017

### BvU

I take it you have googled all the relevant terms, so I am a bit surprised .... etc. What is a terminal velocity if you kick a ball ?

6. Jan 30, 2017

### BvU

But I grant you that you can set up experiments in such a way that you can find values for the variables (in particular Cd) that you need to calculate the terminal velocity. Sensible project, I would say. What measuring equipment do you plan to bring in ?

7. Jan 30, 2017

### CWatters

Fire a ball up vertically at various speeds and measures the speed with which it impacts the ground. Increase the launch speed until the impact speed stops increasing. That impact speed is the terminal velocity.

8. Jan 30, 2017

### jbriggs444

Hang a ball in a wind tunnel using a thin cord. Measure the wind velocity required to get a 45 degree hang angle.

9. Jan 30, 2017

### rumborak

Mythbusters actually did this several times, to measure terminal velocity of objects. They would have a long plastic tube that they pushed up air in upwards, and then they dialed up the air speed until the object was suspended. Might be a bit difficult with a soccer ball though because of the size; you want to have enough space on all sides to not just create a soccer ball cannon.

10. Jan 31, 2017

### BvU

Old vacuum cleaner can be used backwards to create an airflow in which you can suspend a ping-pong ball. Perhaps a hair dryer (I don't have one ) can do the same.

But a ball cannon is so much more fun. Do projectile trajectories with and without friction numerically to interpret your experiments.

11. Jan 31, 2017

### CWatters

I think a leaf blower might work. Use an anemometer to measure the wind speed.

12. Jan 31, 2017

### rumborak

Totally agree of course
Shoot the ball high up in the air, then use a camera to analyze the speed.

13. Feb 2, 2017

### Markus Lervik

Thanks for the help everyone! I will try to shoot the ball high up in the air and use a camera to analyze the speed.

14. Feb 2, 2017

### Markus Lervik

I've decided to drop the ball from a tall building and use the camera to determine the change of velocity. My calculations will look like this:

∑F = ma
F_f=mg=0
C×Vt2=mg

vt= √mg/c

c=(0.5) CD×ρ×A

I would test my numbers by calculating the distance of travel (ball). Then, I would try to shot the ball at the same speed and see if my numbers are right.
Is this a good way to approach the task? Wouldn't there be a significant percentage of uncertainty when I'm testing my results?

Thank you.

15. Feb 10, 2017

### Spinnor

Dangle one from a stick and stick it out the window of a passenger car, speed up to hit your 45 degree angle.

Edit, turbulence is likely to allow only a rough estimate.

Last edited: Feb 10, 2017
16. Feb 10, 2017

### Spinnor

17. Feb 10, 2017