from Come Fly With Me – Exploring Science through aviation and aerospace concepts.

SUBJECT: Science
GRADE: 7,8,9
TIME: 50 minutes
TYPE OF ACTIVITY: Student Investigation
CONCEPTS: Orbit Inertia Gravity
SKILLS: Observation Interpreting Data
Objective: To demonstrate two forces acting on a satellite.

Materials: Five metal washers; 40″ length of string; large wooden spool.


Teacher Background Information:

A satellite remains in its orbit because of two physical phenomena: inertia and gravity. Inertia is the tendency of a moving body to continue to move in a straight line unless something interferes to change its motion. Gravity pulls a spacecraft into orbit around the Earth. Satellites are not slowed by friction because there is almost no atmosphere to cause friction in space.

  1. Push the string through the hole in the spool. Tie four of the washers to one end of the string and tie one washer to the other end.
  2. Ask the class to observe what happens when you hold the spool in your hand and whirl the single washer in a circle. (The four washers should hang down from the bottom of the spool.)
  3. Ask the students why this happens? (The weight of the four washers and the string connecting them with the single washer prevent the single washer from flying off in a straight line as it circles.)
  4. Ask the class to determine what the single washer represents (an orbiting satellite) and what the string represents (the gravitational pull of the Earth).
  5. Ask what would happen if the single washer were able to fly loose from the string? (It would fly out and away in a straight line.)
  6. Ask why a spacecraft does not fly straight into space? (Because of the Law of Gravitation plus retrorockets, attitude thrusters and navigational devices using the laws of motion.)
  7. Ask students to think about why satellites do not slow down and stop? What happens when students run and slide on the ice? What happens when they throw a ball? What is it that causes the skater and the ball to slow down? (Friction) Why doesn’t the satellite slow down? (In space there is very little for a satellite to rub against since space is almost empty.)


For further demonstrations of the everyday experiences which might illustrate straight line inertia of motion ask your students to think about how water is thrown off the wheels of a passing automobile. To demonstrate, stand a bicycle upside down and have a student turn one of the wheels while another pours a few drops of water onto the tire. You can also lay the bicycle on its side and lay small objects on the sidewall of one tire. (Make sure they are small and soft, like an eraser, for example.) Spin the wheel and the objects fly off in a straight line. Ask your students what they think may cause that to happen.

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