Science Buddies: "Ask an Expert"

Hello Jessica,

What kind of material are you using to build the bridges? How many different types of bridges are you going to use? How are you going to build the bridges? Here is a website on different designs of bridges http://pghbridges.com/basics.htm Maybe that will be useful for you to be exposed to more designs.

As you probably already know, a control variable is a quantity that a scientist wants to remain constant. Review this website on variables and hypothesis. https://www.sciencebuddies.org/mentorin ... bles.shtml

Jessica, do you know anything about air pressure? I think the bridges standing alone CAN BE your control. http://kids.earth.nasa.gov/archive/air_pressure/ for more information about air pressure.

Good luck on your project!

Evatseng3,
I am going to make the bridges out of pop-sicle sticks and hold them together with wood glue. I am going to use truss, arch, and suspension bridges. I don't understand how air pressure can be my control. Can you explain it to me? Can the control be how much the bridges weigh to start with? Because that has to be the same for how much weight the bridge will hold. Please reply.

jessica492 wrote:Evatseng3,
I am going to make the bridges out of pop-sicle sticks and hold them together with wood glue. I am going to use truss, arch, and suspension bridges. I don't understand how air pressure can be my control. Can you explain it to me? Can the control be how much the bridges weigh to start with? Because that has to be the same for how much weight the bridge will hold. Please reply.

Dear Jessica,

I think it's a great idea to compare the relative strengths of different bridge designs. As I understand it, your instincts about air pressure are exactly right: as a control for each type of bridge, you can start with the bridge as a stand-alone (i.e. does it hold its own weight and the air pushing down on it?) and then add successive intervals of weight until it collapses to see which design is the strongest. You can think of air pressure as the force (or weight) of all the air that pushes down on the bridge. At sea level, any object--you, me, your bridge--experiences the load of the column of air above it that extends all through the atmosphere. At sea level, air pressure is about 14.7lbs per square inch! You might like to read more about air pressure here:

http://ww2010.atmos.uiuc.edu/(Gh)/guide ... s/def.rxml

Also, I found this article that might help you with your bridge project--it's a bit long, but if you have a specific question, you might try seaching it for "air pressure," which will tell you what you need to know:

http://www.yale.edu/ynhti/curriculum/un ... .08.x.html

Good luck with your project and just post again if you have any questions!

Michael

Skip the air pressure, it is not relevant.

When you build a bridge, the more important variable is the loading pattern. Do you put all the weight (downward force) in one place or distribute it along the length of the bridge. The results are very different.

Robert Reavis

For your control, I would recommend building a simple bridge out a single line of popsicle sticks. (Imagine a log placed over a stream)

Use the maximum weight that this bridge can hold as your control. For your other bridges, build off from the basic control design and create different designs of bridges. You may want to look at <http://en.wikipedia.org/wiki/Bridge#Types_of_bridges> for some ideas on which designs to build.

You should then compare the weights that these other bridges can hold to the weight that your original control bridge held.

I hope this helps