125 Physics Projects for the Evil Genius (6 page)

• 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
  
• glider
  
• 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)
  

1. Attach the string horizontally to two anchor points. The string should be taut and able to support a small weight without sagging.
   
2. Hang the pulley on the string.
   
3. Hang the weight on the pulley so the pulley is free to slide on the string.
   
4. Tape the foamboard or cardboard at an angle of about 20–30 degrees with respect to the direction of the string.
   
5. 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
   .
   

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.
   
2. Attach a fixture on the glider that can hold a flat object, such as a CD.
   
3. Place the CD in the holder and secure it at an angle of about 20–30 degrees with respect to the air track.
   
4. 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
   .
   

Figure 4-1
At the correct angle, the blow dryer will draw the foamboard sail into the wind
.

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.

Figure 4-2
Sailboat simulation using an air track viewed from the side. Photo by S. Grabowski
.

Figure 4-3
Sailboat simulation using an air track viewed from the top. Photo by S. Grabowski
.

The physical structure of a sailboat needs to do at least three things:

1. It picks up the force of the wind (roughly) perpendicular to the sail.

2. The keel of the sailboat makes the sailboat follow one-dimensional motion by preventing the sailboat from slipping perpendicular to its forward movement.

Figure 4-4
Forces on a sailboat
.

3. 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
.

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.

Pressing down with your foot on the accelerator of a car does not necessarily cause you to accelerate. You may be moving forward with constant velocity. How can you tell if you are accelerating? This experiment shows you a few ways to determine whether you are accelerating or just moving along at constant velocity.

In this project, you can also find ways to detect centripetal acceleration, which keeps things moving in a circle.

Any or all of the following “accelerometers” can be used to detect acceleration:

• pendulum: any weight on a string
  
• float tied to a string held underwater
  
• candle
  
• partially filled tank of liquid
  
• accelerometer, such as shown in
  Figure 5-1
  

Figure 5-1
Accelerometer. Courtesy PASCO
.

1. Holding the string of the pendulum, move at as steady a pace as you can. Observe the pendulum during constant velocity.
   
2. Now do the same thing, but observe what happens when you speed up (accelerate).
   

1. Attach a pendulum to a skateboard, as shown in
   Figure 5-2
   .
   
2. Roll it down a ramp that has a large enough slope for the skateboard to increase its speed. Observe the angle the pendulum makes with the vertical position.
   
3. Adjust the slope of the ramp, so the skateboard is just held on the ramp by friction without sliding down. This is called the
   angle of repose
   .
   
4. Give the skateboard a slight nudge. It should move at a fairly constant velocity. Note the angle of the pendulum.
   
5. What happens if the skateboard slows or goes up a ramp?
   

Spin an apparatus, such as shown in
Figure 5-3
or
5-4
. A pair of candles at either end of a spinning board is another way to do this. The floating bob apparatus is commercially available or can be assembled from fishing bobs (or Styrofoam balls), baby food jars, a piece of wood with a hole in the center, and a metal post.