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Centripetal Force Penny

Explore physics and Newton's first law be spinning a coin on the tip of a wire hanger.

As a penny balances precariously on the hook of a wire hanger, you might think any sort of movement would send the penny flying. With a bit of physics know-how, you can spin the entire hanger around in a circle without losing the 1¢ coin. When it comes down to it, you just need to thank Sir Isaac Newton.

Experiment Materials

  • Wire hanger
  • Penny

Experiment

  1. Bend the hook portion of the wire hanger until the end is pointed back in the opposite direction.
  2. Stretch the hanger and open it up. The resulting shape should be very similar to a diamond.
  3. Balance the penny on the hooked end of the hanger. It might take a few attempts, but we promise you'll get it.
  4. Begin to swing the hanger back and forth, starting with a very small amplitude (swing height) and gradually increasing the swing until you can spin it in a full circle. It might take a few times (we'll admit, it definitely took us more than a few) to get it right. But, we promise, it is possible!

How Does It Work?

According to Newton's first law of motion, objects in motion tend to remain in motion unless acted upon by an external force. In this case, Newton's law requires the penny to continue moving along a tangent to the circle. Thus a force is required to keep it always turning toward the center of the circle. The interpretation of this demonstration is potentially confusing when one considers that at the top of its arc, the penny is accelerating downward because of the motion, but that the force of gravity is also downward.

You can explain that the equation Force=Mass x Acceleration (F = ma) is thus satisfied without the penny leaving the hanger.  Force is a push or pull (the swinging of the hanger) that causes an object with mass (the penny) to accelerate.  So, Force of the hanger is equal to the mass of the hanger and penny, times the acceleration given by swinging the hanger. This demonstration provides the opportunity to discuss non-inertial (accelerated) frames of reference and inertial (fictitious) forces (such as the centrifugal force).