Twirler: How Can Gravity Be Used to Power a Machine?

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Author: Janice VanCleave


How can gravity be used to power a machine?


  • scissors
  • shoe box with lid
  • masking tape
  • box of large paper clips
  • empty thread spool
  • pencil
  • modeling clay
  • poster board
  • spool of thread



  1. Cut one of the large sides out of the shoe box.
  2. Place the lid on the shoe box, turn the box upside down, and position it near the edge of a table. Note: The upturned bottom of the box will now be referred to as the box's top.
  3. Tape the box to the table to prevent it from moving.
  4. Open a paper clip and run it through the hole in the empty thread spool.
  5. Bend the paper clip as shown in the of the shoe box with the spool extending past the edge of the table. The spool must turn freely.
  6. Make a hole in the center of the top of the box with a pencil point. The hole must be large enough to allow the pencil to turn freely.
  7. Stick a grape-sized piece of clay to the lid (now on the bottom of the box) directly under the hole in the top of the box.
  8. Cut a circle with a 4-inch (10-cm) diameter from a piece of poster board. Punch a hole in the center of the paper with a pencil point.
  9. Push the eraser end of a pencil into the hole in the center of the other thread spool. The pencil must fit snugly.
  10. Drop the pencil through the hole in the box and then through the hole in the paper circle. Secure the paper circle to the pencil with tape.
  11. Stand the point of the pencil in the mound of clay.
  12. Unwind thread from the spool on the pencil, and pull the thread across the empty spool on the side of the box.
  13. Attach a paper clip to the end of the thread.
  14. Bend the end of the paper clip out to form a hook.
  15. With the end of the paper clip hook just below the empty spool, add paper clips one at a time to the hook until the hook falls.
  16. Observe the movement of the paper circle as the weighted hook falls.


Approximately ten paper clips pull the hook down, causing the spool of thread to unwind; thus, the paper circle twirls.


The twirler is a compound machine (a machine that combines two or more simple machines) made of a pulley and two wheel-and-axle machines. Gravity is the force that pulls things down toward the center of the earth, and in this experiment, gravity is the power source. As the paper clips fall downward, the attached thread is pulled over the spool pulley. The thread spool is turned as the thread is unwound. The turning thread spool also turns the attached pencil, which rotates the paper wheel. Machines enable you to harness forces such as electricity, water power, wind power, or gravity (as was used in this experiment) in order to perform a task. By using different simple machines, the direction, magnitude, and/ or speed of the force can be changed.

Let's Explore

  1. Could the speed of the twirling paper circle be slowed by using smaller weights? Repeat the experiment using smaller paper clips. Science Fair Hint: Compare the weight of the paper clips used in both experiments. The results, along with information from the Check It Out! section, can be used as part of a report about using gravity as a power source.
  2. How does friction affect the speed of falling weights? Repeat the original experiment, increasing friction by wrapping the thread around the pulley spool several times. Then repeat the original experiment again, making the following changes to reduce friction:
    • Glue a button with a large hole to the lid for the pencil point to turn in.
    • Use a thinner, smoother thread.

Show Time!

Construct a gravity-pulled conveyor machine. (Follow the instructions in Experiment 15 for how to build a conveyor.) Create an incline by using a book to raise one side of the machine. Place an object at the top of the inclined machine and record its movement. Increase the incline by adding more books, and repeat the experiment.


Check It Out!

Free-falling objects all fall at the same rate. Find out both what "free fall" means and the speed at which objects fall. When reading about gravity, discover if weight has any affect on the rate at which an object falls.

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