125 Physics Projects for the Evil Genius (14 page)

In this project, you get a pair of ping-pong balls circulating rapidly in a beaker with a blow dryer. The balls continue racing in a frantic high-speed circular path long after the blow dryer is removed. This is a fun, attention-getting demonstration that explores various aspects of circular motion, including angular velocity, centripetal force, and the effect of friction.

• 250 mL glass beaker or plastic container roughly 5 inches (12 cm) in diameter
  
• 2 ping-pong balls
  
• blow dryer
  

1. Place the ping-pong balls in the beaker.

2. With one hand, hold the bottom of the beaker. The other hand holds the blow dryer. (No heat is needed.)

Figure 16-1
Photo by S. Grabowski
.

3. Direct the air from the blow dryer to rapidly circulate the air flow in a circular horizontal pattern inside the beaker.

Figure 16-2
Photo by S. Grabowski
.

4. The blow dryer should get the ping-pong balls to rapidly spin inside the glass container.

Figure 16-3
Photo by S. Grabowski
.

5. As soon as the ping-pong balls are spinning rapidly, quickly turn the beaker upside down and (carefully) place it on the table.

Figure 16-4
Photo by S. Grabowski
.

The ping-pong balls continue to revolve around the inner walls of the container. While spinning, they appear to defy gravity. They also tend to move as far away from each other as they can, especially as they slow down.

The rapidly moving air gives the ping-pong balls kinetic energy. The inner walls of the beaker provide centripetal force that keeps the balls moving in a circular path. The force between the rotating balls and the sidewall of the beaker results in a frictional force that is large enough to hold the balls suspended above the table as they rotate. The balls have enough angular momentum to keep going until frictional forces between the ball and the walls of the container cause them to slow down, resulting in the balls continuing to rotate more slowly and drop to the surface of the table.

Figure 16-5
Photo by S. Grabowski
.

The rapid rotation causes friction between the balls and the side of the beaker. This can cause the ping pong-balls to become charged, resulting in (minor) attraction to the walls of the container and repulsion from each other.

A similar effect can be achieved by vigorously rotating a pair of marbles in an inverted glass or beaker.

The ping-pong balls are given kinetic energy by the blow dryer. Like all rotating objects, their inertia tends to keep them moving in a straight line. The inside walls of the beaker apply centripetal force, which causes the path to be circular. The balls continue to move until the kinetic energy is converted into friction.

If you fill a bucket with water and turn it upside down, the water will (of course) spill out. But, if you spin the bucket over your head fast enough, you may avoid getting wet. How fast do you have to swing a pail filled with water over your head so as to not get wet? In this project, you explore what it takes not to get soaked or, in other words, how fast is fast enough?

• small bucket with a handle or string attached
  
• water
  
• stopwatch
  
• meterstick
  
• person willing to get wet
  
• another person willing to get the first person wet
  
• optional: paper towels or a mop to wipe up spills
  
• optional: raincoat or umbrella
  

1. Put some water in the bucket.
   
2. Predict how fast you think you need to spin the bucket to avoid spilling its contents. This can be done qualitatively by spinning at a relatively fast rotation and pushing your luck by going progressively slower. One simple refinement would be to do this in terms of time (in seconds). A more quantitative prediction can be based on the linear or angular velocity, and it can be measured based on the person’s arm length.)
   
3. Spin the bucket, as shown in
   Figure 17-1
   , and evaluate in terms of the predictions.
   

The slower you go, the greater the risk of soaking the spinner for a given radius. If your arm is shorter, you will have to complete the circle in less time.

Figure 17-1
Spinning a bucket filled with water over your head
.

The maximum time to go around a vertical circle of a given radius without spilling is shown in the table below. The maximum time to spin the bucket overhead is about half that time. Keep in mind these times are based on
uniform velocity
. The most critical point of course is at the top of the circle. (If you slow down there, you may need that raincoat identified in the what-you-need list.)

The person spinning the bucket will be spared a soaking as long as the bucket moves fast enough so the centripetal force is greater than the force of gravity.

The condition for this is:

Note, this result indicates it doesn’t matter how much water is in the bucket as long as the spinner moves at a sufficient speed. The larger the radius, the faster you have to go. Too much water, however, may cause the spinner to slow down.

This can also be done using confetti instead of water.