Introduction
Moving objects appear to glide gracefully through the air or water. But in reality, moving objects are constantly fighting to balance the forces around them. Moving objects have four main forces that act upon them: lift, weight, thrust, and drag (FI, 2006).
Figure 1. The aerodynamics of this airplane are due to the balance of four forces: lift, weight, thrust, and drag (FI, 2006).
The drag on an object is a combination of friction and pressure. Air moving over the surface of an object causes drag due to friction. Higher-pressure air in front of an object pushes back on it more than lower-pressure air behind it, which causes drag due to pressure. The combination of the two is just called "drag," and is also commonly called air resistance. An object moving through water also experiences drag for the same reasons. The force of drag is important for both the aerodynamics and the hydrodynamics of a design.
In this science fair project, you will test how much drag ordinary objects produce when pulled through the water. You will make a simple device to measure the drag by using a pull-scale and fishing line. After testing different objects around your house, will you be able to find out what types of objects move smoothly through the water? Which shapes are the most aerodynamic and the most hydrodynamic?
Terms and Concepts
To do this type of science fair project, you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!- Drag
- Friction
- Pressure
- Air resistance
- Aerodynamics
- Hydrodynamics
Questions
- What causes drag?
- How does drag influence the aerodynamics of an object?
- Which objects will produce the most drag?
Bibliography
- The Franklin Institute. (2006). Forces of Flight. Retrieved December 15, 2006, from http://www.fi.edu/flights/own2/forces.html
- The Franklin Institute. (2006). Forces of Flight - Drag. Retrieved December 15, 2006, from http://www.fi.edu/flights/own2/drag.html
- Andrew Rader Studios. (2006). Motion Basics. Retrieved December 15, 2006, from http://www.physics4kids.com/files/motion_force.html
- Andrew Rader Studios. (2006). Motion: Vectors. Retrieved December 15, 2006, from http://www.physics4kids.com/files/motion_vectors.html
- Sobey, E., 1999. Wacky Water Fun with Science, New York, NY: McGraw-Hill.
- Sobey, E., 1998. Just Plane Smart! Activities for Kids in the Air and on the Ground, New York, NY: McGraw-Hill.
Materials and Equipment
- Scissors
- Fishing line
- Leader line
- Spring scale
- Swimming pool
- Fishing swivels
- Objects to test, with the following features:
- You should use several different types and sizes of each.
- They should not be so large that they are difficult to handle.
- They should all be waterproof, because you will be submerging them in a pool of water.
- They should be objects that naturally sink and not float. For example, choose a baseball instead of an air-filled ball so that it will sink.
- Spheres (baseball, grapefruit, orange, etc.)
- Rectangular shapes (cubes, plastic blocks, boxes, brick, etc.)
- Circular shapes (Frisbee, plate, CD, etc.)
- Irregular shapes (toy, pipe, hammer, jar, boomerang, etc.)
- Lab notebook
- Graph paper
Experimental Procedure
- Select several objects that you want to use for your science fair project, as described in the Materials and Equipment list.
- Wrap fishing line securely around each object. Make a loop out of fishing line at the leading edge of the object and attach a swivel hook. This will be where you attach the leader of the spring scale to each object. Have an adult help you, if necessary.
- Attach a leader line to the spring scale. It should be long enough for your object to reach down into the pool as you are walking along the edge of the pool, and while you are holding onto the spring scale handle.
- Attach an item to the leader line of the spring scale.
- Do a test to measure the drag on the object.
- Hold the spring scale as close as possible to the surface of the water without touching it.
- Then, walk from one end of the pool toward the other end at a constant stride, pulling the item behind you, as it is submerged in the pool.
- It is important to walk at the same speed as you drag each item. Drag can depend on how fast an object is moving, so to make an equal comparison between each object, you need to move them at the same speed.
- You should try to make sure that the leader line is as close to horizontal as possible while you are towing the object. This ensures that the spring scale is mostly measuring drag force (which acts in the horizontal direction), and not any contributions from the object's weight, hydrodynamic lift, or buoyant force, which all act in the vertical direction.
- Look at the spring scale and record the drag on the object (in newtons) in a data table in your lab notebook. Remember to try and walk at a constant pace. The spring scale reading might still bounce around slightly — try to take a middle-value reading if this happens.
| Trial Number | Spheres | Circles | ||||
| Golf ball | Baseball | Cantaloupe | CD | Plate | Trash Can Lid | |
| 1 | ||||||
| 2 | ||||||
| 3 | ||||||
| 4 | ||||||
| 5 | ||||||
| 6 | ||||||
| 7 | ||||||
| 8 | ||||||
| 9 | ||||||
| 10 | ||||||
| Average Drag (N) | ||||||
- Repeat steps 5–6 for the first object, for a total of at least 10 trials to be sure that the results are meaningful.
- Repeat steps 4–7 for all of your other objects.
- Calculate the average drag for each object by adding together the numbers for each object for all 10 trials and dividing your answer by 10. Record all data in your data table.
- Make a graph of your data, either on paper, or with a website, such as Create a Graph. A bar graph will work well for this type of experiment. Make a scale of the force of drag, in newtons, on the left side of the graph (y-axis) and order your differently shaped objects on the bottom of the graph (x-axis). Draw a bar for each item up to the matching force of drag measured in the pool with the spring scale. Be sure you remember to label the axes and the bars of your graph, and to give your graph a title.
- Analyze your data by asking yourself some questions. Which object caused the most drag? The least? If you compare across objects of different shapes, are some shapes better than others? If you compare within a group of similarly shaped objects (for example, all spheres) are some sizes better than others?
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