Science Buddies: "Ask an Expert"

My science fair topic is 'does the air pressure inside a soccer ball affect the distance that the ball will travel in the air' I completed my experiment using a catapult to fire the ball. The results showed no difference in the distance travelled when using different pressures between 1-16 pounds. What could I have done differently and what areas of science affected my results?

thanks for your help

Nick in New Zealand

Hi Soccerfan,

I'm surprised that you got the results you got. In http://wiki.answers.com/Q/Air_pressure_ ... ll_travels
it's stated that "The more air that is in the ball is how far it will travel. For example, a flat ball will not travel as far as a pumped ball. Why? because the bouncyness of the ball would not be used when it is not pumped."

Of course, you can't believe everything you read over the net. Still, you may want to revisit your experiment. It is also possible that using catapults are a special case vs. kicking the ball.

Thank you for your reply. I think you are right about the catapult being the difference. What areas of Science should I be looking at to explain the reason my experiment didn't work and what I should do next time?

From
Soccerfan

Hi Soccerfan,

I’m not sure where you’re heading, keeping this project or looking for a new one. I think comparing distance traveled between kicked and catapulted soccer balls at various levels of inflation and various strengths of the catapult makes a fascinating project.

I think you’re going to have to measure the distance traveled as the initial distance traveled by the ball without bounces or rolls (though you may want to collect this data and see if there is anything interesting.).

Items to note:

1. There is undoubtedly a formula of the distance traveled for a catapulted ball, which explains the results you’re getting. I don’t know what the formula is. You may want to see if you can calculate it, or otherwise find it.

2. Does the angle of the catapult (relative to the ground) impact the distance traveled? If so, it’s another variable to test.

3. It’s going to difficult to consistently measure the distance traveled by a kicked ball under different conditions. You may substitute kicked as hard as you could and hope that the force on the ball is constant.

You may need one or more assistants to measure the initial distance traveled by the ball without bounces or roll.

If the above don’t pan out, you want to look at another project using the topic selection wizard at::

https://www.sciencebuddies.org/science- ... ?From=body

Hi Soccerfan,

The area of science you'll want to talk about to explain this is physics, specifically mechanics (and maybe some ideal gas law), along with some mechanical properties of materials.

So, what happens when you kick a ball or when it bounces is that the kicked area initially bends inward, then the air inside pushes back and it expands outward, pushing off of your foot or the floor. If the air pressure inside is low, then it doesn't push very hard, and by the time it finishes expanding the ball to its original shape your foot might not be there to push off of anymore. Check out these videos:

http://youtube.com/watch?v=7SPmoChvk1w (see around 30 seconds in and around 1 min 15 s in)
http://youtube.com/watch?v=0KfD-v23isA

Some energy is lost when the ball flexes in and out, and at higher pressures this happens to a lesser extent in addition to the ball recovering more quickly and having a better chance at being able to push off your foot. But deformation (shape change) happens even with something as hard as a golf ball:

http://youtube.com/watch?v=2Y57pw_iWlk

With solid (non-air-filled) balls, the ability to bounce or move forward in response to a push (instead of absorbing the force or breaking) is related to elasticity and measured in terms of some elastic modulus. (There are lots of elastic moduli and I'm not sure which one, but if you read more you'll probably find this.) In an elastic collision, no energy is lost -- the ball's kinetic energy after the kick is the same as the energy you put into kicking it (this never really happens, but it's the ideal case). In an inelastic collision, some or all kinetic energy is "lost", or changed into thermal or other energy -- we perceive kinetic energy of air molecules as sound. Basically, when you change the air pressure, you are changing a property of the ball that influences how elastic the collision between the ball and a foot (or catapult) is. Alternately, could you design an experiment that involves bouncing off the floor instead? If you drop it from the same height with throwing it down or tossing it up, the force when it hits the ground will always be the same. It might also be interesting to try take a video of the moment of impact, especially with a high-speed (slow-motion) camera.

So, what is the contact between the catapult and the ball like? If the catapult is basically a cup that covers half the ball, then the force is distributed over the surface and it won't make the ball deform (bend inward) very much (pressure = force/area). When you kick it, all the force is applied over the area the size of the end of the shoe. Could you modify your catapult to do this, maybe by attaching a shoe or object with a similarly shaped end? Also, if the force provided by the catapult is not enough to deform even the least inflated ball, then you won't see much of a difference due to inflation. But I'm pretty sure a smaller force would be needed to do this if you had a smaller contact area, because some of the resistance comes from the wall of the ball and not the air inside.

There could also be a difference in air resistance related to inflation. Air presses on the front of the ball, and a less inflated ball would squash more easily, giving a slightly larger area for air to press on, which would slow it down a bit more. However, air resistance is less of an issue at low speeds, so I think the above explanation is what's happening with your ball.

I'm sure there's much more info out there, and hopefully this is enough to get you started thinking about an explanation and give you some search terms. If you can come up with a good explanation for why it didn't work, and especially if you can repeat the experiment to test this explanation, you could learn more than you initially expected to -- sometimes the most interesting experiments are the ones that don't turn out the way you expect. Please ask more questions if you have them!

Amanda

withOUT throwing it down or tossing it up

(sorry)

Thanks for your reply. I have talked to my teacher and she has told me to stick with the experiment I have done and just explain why it might not have worked and what I could do differently next time. What I am unsure of is how to explain the physics of kicking a ball and the different areas of science that help explain my experiment. I know that gravity and force come into it and some research I have done mentions Isaac Newton's Law of Motion. If you have any suggestions please let me know.

Hi Soccerfan,

In case you missed my response before (more than one of us responded), reading that first will probably make this a little clearer...

Gravity is related to how far the ball will travel given an initial velocity. The velocity will have a component in the horizontal direction, which doesn't change (if you ignore air resistance), and a component in the vertical direction. Because gravity produces a force in the vertical direction, the ball's upward motion will be slower and slower until it stops for just a moment and begins to move downward. Eventually it hits the ground and the horizontal motion stops for that reason. This is interesting and good to understand, but I don't think it explains why your experiment didn't work. You could divide this into two stages:

(a) Why the experiment should work for a kicked ball:

What you're interested in is what determines the ball's initial velocity right after you kick or otherwise propel it. If the kick is the same every time, then this has to do with the elasticity of the collision. Some good search terms would be physics collision or inelastic elastic collision, and here are some good results to get you started:

http://en.wikipedia.org/wiki/Collision
http://www.walter-fendt.de/ph14e/collision.htm
http://hyperphysics.phy-astr.gsu.edu/Hbase/elacol.html

Then you have to think about what determines the elasticity of a ball (and therefore the elasticity of its collision with a foot), and this is where the pressure comes in. This is a really neat bounce simulation where you can change the pressure inside the ball, and it shows an animation with the compression and "efficiency", which is related to the elasticity of the bounce:

http://www.xmas.demon.co.uk/bouncyBall/bouncy.html

This is a paper explaining where the equations used in the simulation come from -- it's pretty technical, so please do ask specific questions about it if you feel up to tackling it:

http://www.iop.org/EJ/article/0031-9120 ... 8120fa091b

There's a chapter on bouncing in this book, some of which is available online -- sorry that it's the wrong sport.

http://books.google.com/books?id=ZZizYIQcZ60C

If you aren't familiar with the ideal gas law, it might also give you some insight:

http://en.wikipedia.org/wiki/Ideal_gas_law

For your purposes, you can probably assume that nRT is constant (that's just the number of air molecules, the temperature, and a constant to make the units work out). When the ball deforms, the volume changes, so the pressure inside the ball has to change. If the volume is divided by two, then the pressure doubles, and so on. So, when the ball deforms, the volume decreases and the pressure inside actually increases, and this is the force that makes the ball move -- otherwise it would just sit there with a dent in it.

(b) Why it didn't work in your case with the catapult:

Without actually seeing a picture of your catapult, I would guess that there are two differences between it and kicking. One is that the force is probably applied over a larger area, so there is less pressure on any given part of the ball's surface. The other is that the catapult probably "carries" the ball through a certain distance to get it up to speed. That means that less force is being applied but for a longer period of time compared to a kicked ball that ends up going to same speed. Think about it this way: Imagine you're standing still wearing roller skates, and a friend pushes you gently from behind until finally your friend is running and you're just in front going the same speed. Now imagine that instead, you're standing still and your friend runs up from behind, pushing all at once to make you go the same speed you ended up going before -- that would probably hurt both of you. A bone in your friend's hand could deform enough to break, or cells in your skin could deform enough to leak blood, resulting in a bruise. You could also think about baseball: The pitcher swings the ball through a long arc to get it up to speed and it doesn't hurt his or her hand, but no one wants to be hit by that ball or to catch it without a glove. In the same way, the ball might not deform much when accelerated over a distance with the catapult, but it would deform pretty easily when kicked quickly.

I think that if you work hard to understand why the experiment should work with a kicked ball, that should answer your questions about why it didn't work with the catapult and what you could do differently. It's awesome that you're persevering with this even though it's gotten complicated!

Amanda

Hi Soccerfan,
It sounds like you have done a good job with your experiment. You had a clear question, designed an experiment to answer that question, and got some sort of answer. I would say that your experiment did work, but you did not get the results that you expected. That is a good thing - unexpected results that do not confirm our hypotheses are what make our "questions" worth investigating, and they are the reason that we do science. As you are discovering, the challenge of unexpected results is that they can be difficult to explain.

So, I don't know how your teacher would feel about this (and you should check) but I would not recommend that you write about "why it might not have worked and what you could do differently next time." I would write about why you think you got the results that you did, and what additional experiments you could do to test your new hypotheses about those results. It is a subtle difference, but it can be an important one to scientists.

I see you have gotten lots of excellent advice. Amanda has mentioned that a catapult probably does not deform a ball as much as kicking it or hitting it with a club. In addition to this, think about what determines how fast a ball moves once it is struck. Kicking a ball is a bit like shooting a rubber band - the more it is stretched, the faster it will fly. There's more to it than that, but I hope that will be enough to get you started.

Regards,
Chris

Thank you all very much for your replies the information has been very useful and I will be using some of it in my project

thanks again

Nick in New Zealand
aka Soccerfan

I’m not sure where you’re heading, keeping this project or looking for a new one. It will really helpful to you.

Light Up Balls