3A40.10-Circular Motion vs. Mass on a Spring


Description: This demonstration shows that Simple Harmonic Motion (SHM) looks like the 2-D projection of circular motion.


  • Circular motion peg apparatus
  • Variable rheostat
  • Large golden spring
  • Set of masses
  • Slide projector
  • Variac
  • 3 banana plug cables
  • Large metal rod
  • Small metal rod with hole at the end
  • S-clamp
  • Table clamp
  • Small stool

Setup Procedure:

  1. Attach the rods to the table. Have it so the spring hangs over the end of the table.
  2. Attach the spring to the hole in the small rod, and attach a mass to the end (around 200-500g).
  3. Place the rotating peg apparatus next to the spring.
  4. Adjust the height of the spring so that the equilibrium position of the spring is at the center of the peg’s height.
  5. Connect the rotating peg apparatus to the variac. Connect the three cables to the variable rheostat. Use a red cable on both sides, and a black cable connected to the black terminal on the rheostat.
  6. Set the variac to about 80V and turn it on.
  7. Make sure moving the rheostat to the faster side actually speeds up the motor. If it doesn’t, reverse the red cables.
  8. Adjust the speed on both the variac and rheostat to match the frequency of the spring.
  9. When in the class, put the slide projector on a small stool and prop it up with a book to project both the spring and peg onto a screen.

Demonstration Procedure:

  1. Turn on the slide projector.
  2. Turn on the variac. Explain that this peg is moving in a cirlce, but the 2-D projection makes it look like SHM.
  3. Move the mass on the spring up and let it go when the peg comes around.
  4. Slight adjustments may be needed on the rheostat to match the two oscillations. In all honesty, you may find it very hard to match the two, but the concept will be shown to the students.
  5. When finished, turn off the variac and slide projector.

Note: This demo is hard to setup and hard to operate.
The belt is very old on the motor. Be careful with the motor.