Science Buddies: "Ask an Expert"

Good day! My son picked a project from this site titled, "What A Drag". The objective is to test ordinary objects for their aerodynamic and hydrodynamic properties by measuring the amount of drag they have. We selected several household objects some of which were spheres, circles, rectangles, etc. We then purchased a digital spring scale and attached a net to hold the objects in place. We weighed each object suspended in air to establish a resting weight and then walked the length of the pool ten times dragging the object under water to measure the force. We then subtracted the resting force from the average force to get our final amount of drag for the object. We didn't realize until now that the scale was measuring in ounces/lbs so we are currently using an online converter to change our ounces to newtons. However sitting down to do the calculations we are coming across a negative number in one of our drag calculations and we are curious if this is possible. For instance the drag of a golf ball in water is coming up to 1.04 Newtons and other objects are coming up with similar results. However one of our heavier objects, a bocci ball (weighing 2lbs) is coming up with a drag of -5.77, is this acurate?? I have done several google searches to find the answer however am not coming up with an answer. I have spent several days helping my son convert all the results done in ounces to newtons and now we have come across this and he is beginning to find this project more frustrating than exciting. We want to jump back into the rest of the project however I would like to make sure that a negative drag is possible before we continue. Thank you in advance for any help you might be able to provide with this! I really aprreciate it

Hello and welcome to the forum.

First, it is possible that using a net instead of connecting the objects directly to the fishing line could be causing some of your problems. It is possible that for the smaller items you are measuring the drag of the net as well as the object, and this may throw off your results.
Next, is it possible the bocci ball is hollow? I imagine that it sinks, but if it contains air it may influence your results.
And no, you should not get a negative number. I'm a little curious about this experiment, as you measure the weight of the ball in the air, but the drag with the object under water. This may be the reason for the problem. I will ask the other experts what they think. Please let us know if you find out anything.

Keith

Thank you Keith! We are going to conduct the experiment again tomorrow using the fishing line. The reason we had tried a net was because it's almost impossible to secure fishing line to certain objects (plate, bocci ball, etc.) I was also curious why the experiment called for weighing the items suspended in air but then all other measurements being done in water. We are starting to run out of time though so I'm beginning to think we might need to try another experiment. Thank you again and hopefully tomorrow will be better!

Hi:

I heard from an expert on this project at Science Buddies, and they are updating the experiment based on your findings (thanks!) Here's what they said:

just measuring the object's weight when it's hanging vertically from a scale, and subtracting that weight from the scale reading when towing the object to calculate drag, doesn't make any sense and will give incorrect results - possibly even resulting in a negative number, as the user stated

Also, you want to be sure that the fishing line is as close to horizontal as possible so that you measure only the drag - if it's much off of a line parallel to the water surface, you would have to use trigonometry to calculate the drag.

Let us know how it turns out!
Keith