It is not hard to understand how a good stiff wind blowing from behind a sailboat can move it along at a brisk pace in the water. But what about getting back home? How can a sailboat move (or tack) against the wind?
In this project, you discover how a sailboat moving against the wind can result in a force that pushes the sailboat forward. This gets to the idea of how a force in one direction can be broken down into separate component forces. Two methods are shown here. The first method uses a sail attached to a pulley on a string. The second method uses an air track for those readers who have access to one. After looking at these methods, you are encouraged to try one or both of these, or to come up with your own idea.
What You Need
Pulley and string:
- stiff piece of foamboard or cardboard to use as a sail
- (low-friction) pulley
- small mass with an attachment hook, approximately 20 g
- 1–2 meters of thin string
- attachment points (such as ring stands clamped to a lab table) to hold the string horizontally
- blow dryer or other source of air flow
- duct tape
- air track
- attachment for the glider that can hold a "sail." A bumper, for instance, can be attached to the top of a glider to serve as a "mast."
- 1 CD (or a stiff sheet of cardboard)
Pulley and string
- Attach the string horizontally to two anchor points. The string should be taut and able to support a small weight without sagging.
- Hang the pulley on the string.
- Hang the weight on the pulley so the pulley is free to slide on the string.
- Tape the foamboard or cardboard at an angle of about 20–30 degrees with respect to the direction of the string.
- With the sailboat supported on the string, direct the blow dryer at the sail. The blow dryer should be at a slightly greater angle (with respect to the string) than the angle of the sail. If the air from the blow dryer is too strong, the sail may vibrate. If the angle is too small, the sail will be forced backward with the wind. However, under the right conditions, the force in the forward direction will be strong enough to propel the pulley against the wind, in a similar manner to a real sailboat. See Figure 4-1.
- Level the air track. You can determine that the air track is level by observing the glider when the air track is activated. If the glider does not move in either direction under the force of gravity, then the track can be considered to be level.
- Attach a fixture on the glider that can hold a flat object, such as a CD.
- Place the CD in the holder and secure it at an angle of about 20–30 degrees with respect to the air track.
- Direct the blow dryer at a slightly greater angle than the angle of the sail, and then observe its response. See Figures 4-2 and 4-3.
For either method, the action of the blow dryer if positioned properly causes the "boat" to move toward the blow dryer. The boat is seen to move "against the wind." The parallel component of the force will cause the sailboat to move forward or tacking against the wind.
Using the pulley, if conditions are right, the perpendicular component of the force will also cause the sailboat to rotate around the string. This is comparable to a sailboat listing under the force of a strong wind. The keel of an actual sailboat serves to counteract the effect of this perpendicular force. In this experiment, this force is not constrained and causes the pulley to rotate.
Why It Works
The physical structure of a sailboat needs to do at least three things:
- It picks up the force of the wind (roughly) perpendicular to the sail.
- The keel of the sailboat makes the sailboat follow one-dimensional motion by preventing the sailboat from slipping perpendicular to its forward movement.
- It separates the force of the wind into two parts: one perpendicular to the movement of the boat, which is resisted by the keel, and one parallel to the motion of the boat, which propels it forward.
Figure 4-4 shows how the forces are separated into two components. The force produced on the sail by the wind blowing gets split up by the sailboat into two other forces. One tries to push the boat sideways and is resisted by the keel. The other force—if the angles are right—tries to push the boat forward. This happens even if the wind is coming more from in front than from behind. Quantities in physics that can be broken down into components as this force on a sailboat are called vectors.
Other Things to Try
Attaching a foamboard or cardboard sail to a toy car will work. The wheels of the car must turn freely and the tires must have enough friction to serve as a "keel" to restrict sideways motion.
Another way to do this is to use a (nearly) frictionless hockey puck with a low-friction tube to constrain motion in one dimension. A guide string (such as fishing line) is used to keep the motion in one dimension. You have to keep enough tension on the string to prevent the puck from rotating and binding. The puck must also be on a nearly perfectly horizontal surface. Tape a sail as in either of the two methods previously described. This approach also requires a reasonably horizontal surface to prevent the puck from drifting on its own before the blower is turned on.
A force in one direction can be thought of as being equivalent to two other forces pushing in completely different directions. This happens because force is a vector quantity in physics. This project illustrates how a force on the sail of a sailboat is the same as a sideways force pushing against the keel and a force in the forward direction of the sailboat. This is an example of the resolution of a force into two perpendicular components.