The Spinning Penny Warning: The spinning penny trick is known to be addicting. email to friend
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Squeeze a penny through the mouth of one of our clear balloons. Make sure that the penny goes all the way into the balloon so that there is no danger of it being sucked out while blowing up the balloon. Blow up the balloon. When properly inflated, the balloon will be almost clear in the middle and cloudy at area near the neck and at the end opposite the neck. The cloudiness at the ends is unstretched latex, which provides stress relief. If the balloon is completely clear, all over, it is over inflated. Tie off the balloon and you’re ready to go. Grip the balloon at the stem end as you would a bowling ball. The neck of the balloon will be in your palm and your fingers and thumb will extend down the sides of the balloon. While holding the balloon, palm down, swirl it in a circular motion. The penny may bounce around at first, but it will soon begin to roll around the inside of the balloon. The best orbit or path for the coin is one parallel to the floor. Once the coin begins spinning, use your other hand to stabilize the balloon. Your penny should continue to spin for 30 seconds or more.

How does it work?

Use this demonstration to pique curiosity about centripetal forces. Centripetal force is the inward force on a body that causes it to move in a circular path. The old concept of “centrifugal force” (an outward or center fleeing force) has been largely replaced by a more modernistic understanding of “centripetal force” (an inward or center seeking force). When we attach a ball to a string and swing it in a circular path, we feel the forces of the ball pulling on the string, and that of the string pulling on our hand. That effect is probably responsible for the misconception of a centrifugal, or center fleeing force. Scientists have now proven that only centripetal forces are responsible for the effects we experience with the ball and string. Let’s consider the experiment where one swings a tethered ball in a circular motion around one’s head. If we were to let go of the string, and if center fleeing forces (centrifugal) were in effect, the ball would fly off radially from the point of release. But, it doesn’t do that! Instead, it flies off tangentially, in the direction of the velocity it had at the moment it was released. In the example of the ball and string, it is the string that supplies the inward force while in the case of the penny in the balloon; it is the balloon that imposes an inward force on the penny thereby keeping it traveling in a circular path.

Additional Info

Compare the motion of the penny in the balloon with that of the planets around the sun. See if your kids can figure out that gravity takes the place of the string and balloon in providing an inward force on the planets. Compare the behavior of a gyroscope to that of the penny spinning in the balloon. A gyroscope is essentially a spinning mass, and so is the penny. Once the disk (mass) of the gyroscope starts spinning, it resists tipping on its axis of rotation. A child’s spinning “top” is a good example of a gyroscope to which we can easily relate. Just as the “top” resists tipping over while it’s spinning, so does the penny. The gyroscopic action of the penny provides stability to its orbit within the balloon.