Materials

• Ball-point pen with ink cartridge removed
• One-metre long wooden ruler
• Tape
• About two metres of string
• Rubber band
• 2 rubber balls (tennis balls are fine) and a mesh bag
• Metronome or watch with second hand

Instructions

1. Tape the pen barrel flat to one end of the ruler, with the pen end extending past the ruler's edge. Run the string through the pen barrel. Tie the rubber band to the string end that runs down the length of the ruler. Tie the mesh bag to the string at the pen end and put the tennis ball in the bag.

2. Hook the rubber band around the end of the ruler. Now swing the ball around above your head. Read the centripetal force in terms of the length of the rubber band.

3. Set the speed of your turns with the metronome, or have someone call out times with a watch. Adjust the speed of the ball until it passes exactly the same point with every tick of the metronome, or time called out. The orbital speed is the number of revolutions made per second.
   
   
4. Measure how much the rubber band stretches (the force) when the orbital speed and distance is kept constant. Repeat the trial. This is the control, a standard against which other experimental results are measured.
   
   
5. Measure the force when the orbital speed is exactly doubled. Keep everything else the same as in the control. Repeat the trial.
   
   
6. Remove enough string so the orbit is half what it was in the control. Then measure the force when the orbital speed is the same as in the control. Repeat the trials.

What's Happening?

Mass, distance, and speed all affect centripetal force. You can determine how each affects the force by looking at your data after you change the variables. What is the effect on the centripetal force when the mass and speed are constant but distance halved? What is the effect on the force when the distance and mass are constant but speed doubles? You can change the mass by adding a second tennis ball. See what happens to the force then!