Module 3: Orbits and Systems


Drawing Orbits - Directions:

Draw an ellipse of the right orbital shape:

  • for an imaginary comet with orbital eccentricity 0.50,
  • for Earth
  • for "your" world

The technique uses thumbtacks and a string, and can be found at: Drawing Orbits

Turn in:

1. The placement of pins for each ellipse

2. A description of the resulting shapes. If you use a technique other than the one given in the link, describe the technique, the measurements used, and the resulting shape. (You obviously won't be turning in your actual drawings.)

Dancing Orbits - Directions:

Swing a partner (or pole) and pull in to gain a visceral understanding of orbital energy and momentum.

1. Get a partner. Alternative: go outside and use a lamppost or other solidly fixed pole instead of a partner. (You may need to wrap a rope or belt around the pole to get enough of a swing.) This is fun to do with an adult and child.

2. Brace your left feet against each other, side by side, outer ankles near each other. Hold hands or wrists.

3. Lean outward (REALLY) until you have to pull with your hands to stay up. Continue leaning out and maintaining tension in your grip. This is important.

4. Circle counterclockwise, using your left feet as a pivot, and pushing with your right feet. (Put down a piece of paper under your pivot feet if your shoe soles grip too much.) Try to go at a SLOW but even pace, keeping your arms mostly unbent. Keep leaning outward.

5. Now: both together (still leaning outward), pull harder with your arms so you move closer as you continue going around. What happens to your motion? Repeat several times - it's unusual to be able to pay much attention the first time.

This experiment is related to "conservation of angular momentum" and also conservation of energy (potential = stored energy and kinetic = energy-of-motion) in an orbit. How does it illustrate one of Kepler's laws? Think about the orbit of a comet to help you.

Turn in answers to these questions:

  • What happened when you pulled in?
  • Were your motions around each other more like a comet or planet and Sun, or more like Pluto and Charon? Explain briefly; it may be useful to estimate the ratio of your masses.

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Montana State University

last updated 02/10/02