Abstract

Objective

To determine which mountainboard tire pressure provides the best speed, and which mountainboard tire pressure provides the best control.

Introduction

Pressure. You feel it all the time, just by living on Earth. At sea level, you have nearly 15 pounds per square inch (psi) of pressure pushing on you from Earth's atmosphere, that "air ocean" that you breathe and live in. Did you know you were being squeezed so much? Your body thinks it's normal!

So, what is pressure? To understand that, you first have to know what a force is. A force is a push or pull that can make an object change its velocity. For example, when you roll a ball along the floor, you give it a push to make it change from standing still to rolling. When describing pressure, you don't talk about any old force though. You only consider the force that is perpendicular to the surface of the object. This is known as the normal force. Pressure is defined as force per unit area. The word per tells you that you use division to calculate pressure. You divide the normal forces on the surface of an object by the surface area to get pressure.

You've had lots of everyday experiences with pressure. For example, if you are going to cut an apple and you try to cut it with the flat side of a knife, as shown below, you can't even break through the skin, but if you turn the knife so that the blade's edge is turned into the apple, you will cut through with relatively little force. What changed when you turned the knife? The surface area. When the knife blade is held flat, the surface area is large, making the pressure on the apple low. When the knife is held with the thin blade turned into the apple, the surface area is much smaller, making the pressure on the apple much higher, and a cut can be made. The same is true if you think about your thumb and a thumbtack. Press on a wall with your thumb, and there is no damage to the wall. You cannot make a hole. But if you put a thumbtack under your thumb and press, you will make a small hole in the wall because the thumbtack point creates a very small surface area, raising the pressure applied to the wall.

Figure 1. This drawing shows that when you change the surface area of the knife that is in contact with the apple, you change the pressure on the apple.

A knife and a thumbtack are examples of solids, but liquids and gases, like the water and the atmosphere, are also capable of applying pressure. For example, when you go beneath the ocean to a depth of 1 mile, you have 2,270 pounds per square inch of pressure upon you. That's because a mile-long, 1-inch-square column of water weighs 2,270 pounds (lbs). That's more than a ton! No wonder divers need specialized hard suits and diving vehicles. Up on dry land, the pressures are much more comfortable at about 15 psi. That's because Earth's atmosphere extends about 50 miles outward from the Earth, and a 50-mile-long, 1-inch-square column of air weighs only about 15 lbs. Whew! Aren't you glad you live in an "air ocean" rather than the real one?

Click here to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org.

If you corral air inside a container, like a balloon or a tire, you can create air pressures inside the container that are different from those of the atmosphere outside the container. The atoms of air inside the container are in constant motion, bouncing all over and putting pressure on the walls of the container. You can increase the pressure on the walls by increasing the temperature of the air so that the atoms move faster and bang into the walls more frequently, or by adding more air atoms to the container.

On a hot summer afternoon, you might notice that your bike tire feels firmer than it does in the morning when it is cooler. That's because the atoms have been warmed up and have more motion. They are hitting the sides of the container more frequently. When you add air to a tire with a pump, you also make your tire firmer. The pump is adding air atoms to the inside of the tire. Increasing the number of air atoms increases the number of collisions on the wall of the tire, which increases the tire's pressure.

In this science fair project, you'll see what happens when you adjust the tire pressures of your mountainboard. At lower pressures, around 10 psi, the tires will feel squishier, and there will be more tire surface area in contact with the ground. This may improve handling and control of your mountainboard, because there is more of the tire gripping the trail, but it might also slow you down, as a greater surface area results in greater friction, or resistance to motion. At high pressures, around 60 psi, your tires will feel firm and hard to compress. There will be much less tire surface area in contact with the trail, so there will be less friction. This may give you great speed, but a lack of handling and control, too, which might make you unable to finish your course. Wipe out!

If you'd like to get a preview of this gnarly science experiment, visit the DragonflyTV video on the right and join Sean, Ben, and Neil as they shred some mountain in southern California.

Terms, Concepts, and Questions to Start Background Research

• Pressure
• Pounds per square inch (psi)
• Force
• Velocity
• Perpendicular
• Normal force
• Per
• Division
• Surface area
• Solid
• Liquid
• Gas
• Atom
• Friction
• Inverse relationship

Questions

• What is a normal force?
• What is surface area?
• How is pressure defined?
• How can you increase or decrease air pressure within a container like a balloon or a tire?

Bibliography

This science fair project was inspired by this resource:

• TPT. (2006). Mountainboarding by Sean, Ben, and Neil. DragonflyTV, Twin Cities Public Television. Retrieved September 17, 2008, from http://pbskids.org/dragonflytv/show/mountainboarding.html

This source discusses tire pressure, its importance, and how to properly measure it:

• Wiesenfelder, J. (2005). Car Talk Service Advice: Pressure. Retrieved September 18, 2008, from http://cars.cartalk.com/content/advice/tirepressure.html

This source describes the concept of pressure, as well as the mechanics of a tire gauge:

• Brain, M. (2008). How Tire Pressure Gauges Work. Retrieved September 18, 2008, from http://www.howstuffworks.com/pressure-gauge.htm

This source describes what air pressure is:

• Sample, S. (2003). It's a Breeze: How Air Pressure Affects You. Feeling Pressured? National Aeronautics and Space Administration. Retrieved September 18, 2008, from http://kids.earth.nasa.gov/archive/air_pressure/index.html

Materials and Equipment

To do this science fair project, you must have access to a hill or mountain suitable for mountainboarding, all mountainboarding equipment, and all protective clothing and safety gear. The test will likely be more accurate if you are adept at mountainboarding.

• Tire gauge, capable of measuring pressures between 0 and 60 psi. If possible, purchase an analog-dial or a digital tire gauge, which tend to be more accurate than the ballpoint pen type of tire gauges.
• Tire pump
• Sports cones; available at stores that sell soccer supplies (8)
• Helpers (2)
• Cell phones or walkie-talkies (2)
• Stopwatch
• Lab notebook

Experimental Procedure

Preparing Your Test Course

1. Put on your protective clothing, safety gear, and helmet.
2. Using the tire gauge, check the pressures of each tire, and add or release air until they all read 25 psi.
3. Have your helpers time you making some test runs on your hill until you find a course that takes you approximately 20–30 seconds to complete.
4. Start at the top of your course and mark a starting line in the trail with an item, such as a stick.
5. Walk down the course about 20 paces and place a cone on the trail. With your shoe, mark a little hole under the cone, so that if the cone gets knocked out of position, you'll know where it was for the next test run.
6. Walk down another 5 paces and place another cone, again marking a little hole in the trail under the cone with your shoe. You may need to experiment to see if 5 paces is too close or too far. You want the cones to be close enough together that you are challenged as you try to make the turns from one to the next, but not so close together that you are knocking them over every time you make a run.
7. Continue down the course, setting up a cone about every 5 paces, and marking their location underneath, until all the cones have been set up.
8. Record the course information, such as number of cones and their distance apart, in your lab notebook.
9. Mark the finish line for your course with another stick.

Running Your Mountainboard Tests

1. Check your tire pressures again with the gauge to make sure your tires are all still at 25 psi.
2. Station one helper at the top of the course with a walkie-talkie or a cell phone. Station another helper at the bottom of the course with a stopwatch (that has been set to zero), the other walkie-talkie or cell phone, and with your lab notebook and a pen.
3. Put on your protective clothing, safety gear, and helmet and wait at the starting line for your helper beside you to give the command to start.
4. The helper at the starting line should speak into his or her walkie-talkie or cell phone so that the helper at the bottom can hear when the command to start is given. When the helper at the starting line says, "Ready, set, go!" the helper at the finish line should start the stopwatch, and you should start your trial run on your mountainboard.
5. You should attempt to slalom (zigzag) between the cones without knocking them over.
6. When you cross the finish line, have the helper at the finish line stop the stopwatch and record your time in a data table, like the one below, in your lab notebook for the current trial. Have your helper at the starting line report how many cones were knocked out of position or knocked over. Record that count in your data table as well.
7. Reposition any cones that were knocked over or out of place.

8. Repeat the section "Running Your Mountainboard Tests," steps 1-7, two more times so that you have three trials at 25 psi.
9. Adjust the pressure on your tires to 10 psi, and repeat "Running Your Mountainboard Tests," steps 2-7, three times, so that you have three trials at 10 psi.
10. Adjust the pressure on your tires to 60 psi, and repeat "Running Your Mountainboard Tests," steps 2-7, three times, so that you have three trials at 60 psi.
11. Calculate the average times to complete the course for each psi setting, and the average number of cones out of position for each psi setting. Record the averages in the data table in your lab notebook.

Analyzing Your Data

1. Make two line graphs. For one line graph, plot Tire Pressure (in psi) on the x-axis and Average Time to Complete the Course (in sec) on the y-axis. On the other graph, plot Tire Pressure (in psi) on the x-axis and Average Number of Cones Knocked Out of Position on the y-axis. Which tire pressure gave the best mountainboard control? Which tire pressure gave the best mountainboard speed? Was there an inverse relationship between speed and control? As speed went up, did control go down?

Variations

• Repeat this science fair project on test courses with different surfaces, such as sand and asphalt, to determine the best tire pressure for each surface.
• Repeat this science fair project with a mountain bike to see if a two-wheeled ride requires different tire pressures than a four-wheeled ride for optimal speed and control.

• For more science project ideas in this area of science, see Sports Science Project Ideas.

Credits

Kristin Strong, Science Buddies

Last edit date: 2008-11-18 16:54:00