Science Buddies: "Ask an Expert"

My project is about finding the drag of my objects. Drag is the push you feel when you run. So I have two problems. One, I don't know what the questions the judges are going to ask me. Two, one of my objects, the soda bottle,the brick, the vase, and the hammer, has the most drag and that is the soda bottle. But one thing, I thought it was suppose to have the least drag compared to the brick. So i need help. PLEASE send me a reply by tomorow

Hi quangnguyen,

Often a judge will ask you to tell him or her about your project, and they might interrupt your presentation when they have questions, or they might save questions for the end. The basic idea is to explain how the scientific method is implemented through your project, so you'll want to explain the background information that led to your initial question, the hypothesis, the variables (independent, dependent, and controlled), how you carried out the experiment, the results you obtained, and how you interpreted the results to form conclusions. When I judge science fairs, I usually also ask students how they became interested in their topics and what they would like to change or extend about their work if they continued with it.

These resources should be helpful:
https://www.sciencebuddies.org/science- ... ging.shtml
https://www.sciencebuddies.org/science- ... tips.shtml
https://www.sciencebuddies.org/science- ... ndex.shtml

Good luck!
Amanda

Thanks Amanda,
But is still have that other problem. I seriously need to find out why. Thank-you though.

OK, you edited your post while I was responding. It's usually a better idea to make a second post if you're changing the content (rather than correcting a spelling error, etc) since people will think they've already read the post. And those of us who receive email alerts for new posts don't get them for edits.

Anyway -- we really need more information to try to help you figure out what's going on. How did you measure the drag force? Were your objects moving through air, water, or another fluid? What was the reasoning behind your predictions regarding which objects would experience more drag than others?

Looking at the basic drag equation (http://en.wikipedia.org/wiki/Drag_equation), there are several variables factoring into the force. The density of the fluid should be constant if you always used air or always used water, etc. Hopefully you set up your experiment so that the relative velocity of the fluid and the object was always the same, but this depends on how you made the measurements. The last two, area and drag coefficient, are complicated. The area is usually close to the cross-section of the object, but the drag coefficient (http://en.wikipedia.org/wiki/Drag_coefficient) depends on the geometry of the object. Fluid dynamics is complicated -- the path that a fluid takes around an object moving through it, and the resulting force against that object, depends very strongly on the shape of the object. For example, the soda bottle probably experiences less drag when the end with the cap is pointed forward than when the bottom is pointed forward (look at the shapes in the last link).

What I'm concerned about is that the set of objects you describe have both different cross-sectional areas and different geometries. It's easiest to interpret results when only one variable changes, otherwise you don't know which one caused the change. Were you able to make one of those variables constant? I can imagine that you found a vase, soda bottle, and brick with similar cross-sectional areas, but I doubt there are many hammers that size... if you weren't thinking about this, and were basing your predictions on the objects' shapes (drag coefficients) only, you might be able to convert your measurements to drag force per unit cross sectional area and compare those.

Amanda

Thanks Amanda,
I measured the drag in the water. But I measured the wieght and then i put it in the water, dragged it around the pool, and measured in nwetons ten times.Sorry for the misunderstanding. But the websites have way too many big words for my grade. I am in 4th grade. Please send easier ones. And oh i did find a hammer that size and my experiment tod me to the experiment based on shape. It's called "what a drag"










Thanks
quangnguyen

For others' reference, the project: https://www.sciencebuddies.org/science- ... p022.shtml

A few questions that you still haven't answered...

-- What device did you use to measure the force that you pulled with? How was it connected to the objects?
-- When you predicted which objects would experience the greatest drag force, what was your reasoning?

Additionally...

-- Are you sure that you were always walking at the same pace? For example, did you time yourself over a certain distance with a stopwatch?
-- It's great that you repeated each measurement 10 times. Were the 10 measurements for a given object fairly similar, or did they vary a lot? (This is a good indication of the precision of your measurement technique.)
-- Have you seen the drag equation before? If not, what was your understanding of drag when you formulated with your hypothesis?
-- Which sides of the objects were pointing forward? (cap end or bottom of bottle, bottom or top of vase, etc)

Basically, the drag equation says that the drag force F is proportional to several quantities. One of them is the density of fluid, which you kept constant by always using water. Another is the square of the velocity -- if you were able to walk at the same speed every time, then this was also constant.

F is also proportional to an area A, which it's reasonable to approximate as the cross-sectional area of the object -- the area that's perpendicular to the direction of motion. For example, if you had a brick that was 4 inches by 5 inches by 8 inches, and you dragged it with the smallest side facing forward, then A would be 4 in * 5 in = 20 square inches. The area is important because a larger area means that more water has to be displaced as the object moves.

The last variable to which F is proportional is the drag coefficient C. This depends on the geometry, or shape, of the object. Rather than the shape of the cross-sectional area, it's the shape of the side profile that's important. Seriously, look at the shapes in the diagram in the upper right here -- the objects are moving in the direction of the arrows: http://en.wikipedia.org/wiki/Drag_coefficient. The basic idea is that a curved or pointy surface deflects the fluid gradually, so it doesn't hit the moving object in a head-on collision and doesn't push as hard against it. The drag coefficient isn't something you can calculate -- it's usually measured experimentally.

When you design an experiment, it's best to have one independent variable (the thing you change intentionally) and one dependent variable (the quantity you measure -- here, the drag force). Everything else should be kept constant, or you won't know whether your IV caused changes in the DV or something else did. The problem with your setup might be that you have two important variables that are changing: cross-sectional area and drag coefficient. Comparing just the bottle and the brick, it's reasonable to predict that the cap end of the bottle would have a lower drag coefficient than any side of the brick, because flat is pretty much the worst surface, and the curved/pointy top of the bottle is much more similar to the 'streamlined body' shape. However, if you used a 2-liter bottle, it's likely that the bottle's cross-sectional area is somewhat larger than the brick's. Depending on the magnitudes of the differences between the areas and the drag coefficients (and the accuracy of your measurements), this could explain how you ended up with a larger drag coefficient for the bottle.

Ideally, the experiment would have been designed so that either (a) all objects had the same area, or (b) all objects had the same drag coefficient. (b) is tricky unless you use a simple geometry like a flat surface -- bricks of different sizes would work. (a) could be accomplished by something like measuring the two different ends of a bottle, or comparing a plate and a ball of the same diameter.

Assuming that you don't have time to make more measurements, here's an idea. So, you have the equation F = constant * A * C. You've measured F, and you can also measure A. If you want to "get rid of" A to test whether C corresponds to the object's geometries in the way you'd expect, you can divide both sides of the equation by A to get the drag force per unit surface area (might want to measure area in metric units since you used Newtons): F/A = constant * C.

My standard warning about Wikipedia (since I've linked there) is that it's a great first stop for looking up new terminology and finding related concepts, but it's not a primary source, and most teachers probably don't want you to cite it (for information -- of course you have to cite it if you borrow an image, etc). I suggest plugging terms like 'drag force', or anything else you don't understand, into Google or another search engine to find sources that make sense to you. You should have done some background reading before starting your experiment, so this should follow as an extension to that information.

Amanda

Thanks for ansewering Amanda,
But can you make brief? Anyway, i used a spring scale to measure. I conected a fishing line to the spring scale.I predictied the brick becauese of it's weight. At first i didn't know what drag was so I just predicted the brick. I was walking at the same pace. All of the measurement was close.Everyone of them was faced forward except the brick. By the way, I looked in every reasearch website i know but didn't quite understand it. But it's all thanks to you know that I understand it.








Thanks for your hard work my mom and i apperciated. Love, Quang

Glad I was able to help somewhat. I'm sorry if you got into something that was a little over your head -- fluid dynamics is an advanced subject! -- but I bet they'll be impressed that you tackled something difficult. I found a couple of websites that might be worth looking at...

Some info from NASA directed at a K-12 audience (maybe more high school, but at least not assuming you're in college!):
http://www.grc.nasa.gov/WWW/K-12/airplane/drag1.html
http://www.grc.nasa.gov/WWW/K-12/airplane/dragco.html

Notes from a college course, but has good diagrams of fluid flow paths and a chart of drag coefficient for different shapes:
ftp://ftp.colorado.edu/cuboulder/course ... agCoef.pdf

Could be used for some calculations if you have time:
http://www.lmnoeng.com/Drag/index.shtml

Good luck at your science fair tomorrow!
Amanda

Thanks for everything,
Thank yoo Amanda. Everything now makes perfect sense. You helped learn everything about drag.

Thank you,
your friend quangnguyen

Hi Amnada,
Thank you for everything you've done for me. Wish me luck with the science fair tommorw but i have one more problem . How can i make myself stop being so nervous!? I keep on being so nervous. Please send some tips tonight.




Thanks for everything and your hard work,
Your friend quangnguyen

Hi,

I used to be a really nervous person too! What helped me most was just to give lots of presentations--eventually you just kind of realize that it's no big deal and stop getting nervous (at least you stop getting AS nervous...). BUT, in the meantime, here's a few suggestions:

Take a deep breath before you start to talk. Remind yourself that you can do this.
It's okay to tell the judges that you're a little bit nervous--sometimes it helps to just admit it!
Remember that the judges are honestly interested in what you're going to say. They're not going to make fun of you or be angry or mean. If you can try to talk to them like you talk to your friends, it may help.
Some people say that picturing judges in their underwear helps. Personally, this is not something that helps me any.
Try to think of some similar things that you've done (have you explained the experiment to your teacher? To your parents? To your friends? Have you been to a science fair before? Have you given a talk in front of an audience before?). Then, remind yourself that you got through all of those events just fine and you'll get through this one the same way. You are capable of doing this!
Last but not least, HAVE FUN! The judges are much more likely to respond positively if it seems as if you're enjoying yourself. I know it can be hard to have fun when you're nervous, but it is actually possible to be both at the same time.

I am sure that you will do a great job and represent yourself well. Good luck!

Hi everyone,
THank-you for everything you've done for me everyone. Everybody's information helped me with my project. I thank-you for that.You're info made me get second place.Not bad for a begginer,right? Mostly I would like to thank-you Amnada. Amanda, if you're reading this, you're the most resourceful person. You're the main reason why i got second place. I thank-you for that. My mom and i apprecitate you're time for us. Melisa, you helped me calm down. I also thank -you for that. You helped me answer questions fluently. Thank-you.








Your appreciated friend,
quangnguyen

Congrats on winning second place!

Amanda

Congratulations, and you're very welcome!