# 20. INERTIA & GRAVITY – CIRCULAR ORBITS

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

SUBJECT: Science
GROUP SIZE: Small
TIME: 50 minutes
TYPE OF ACTIVITY: Student Investigation
TEACHING STRATEGY: Guided Discovery
CONCEPTS: Orbit Inertia Gravity
SKILLS: Observation Experimentation

Objectives: To explore the forces necessary to achieve a circular orbit using a model; to learn in what direction, relative to gravity, inertial force must be applied to achieve a circular orbit.

Materials: Two ropes or cords about 2-3 meters long; a box or chair; the basketball set up from the Oval Orbits activity # 19.
Teacher Background Information:

To achieve a true circular orbit, a space vehicle after launch must use rocket thrust aimed at right angles to the center of gravity of the earth. The following activity demonstrates the phenomena in two ways: first, using a demonstration with two ropes and a weighted box or a chair and, second, with the basketball and string set-up from the previous activity.

Procedure:

1. Tie two ropes to a box or chair.
2. Have two students hold the ends of the ropes and move away from the box or chair at right angles to each other.
3. Have the two pupils pull straight back slowly on the ropes. Note the direction the box or chair travels relative to the students.
4. Have one student, and then the other, pull harder on their ropes. How is the direction of the box or chair changed?
5. Try angles other than 90° and note the angle of the path the box or chair takes. Explain to the students that one rope represents gravity and the other rope represents inertia from the rocket.
NOTE: To achieve a specific direction somewhere between the direction of thrust and the direction of gravity the two must be at right angles. Too small an angle will result in a trajectory which will carry the satellite (box or chair) back to earth. Too great an angle will result in the satellite losing speed and then falling back to earth unless it can achieve enough speed (40,000 kph) to break away from gravity.
6. Using the basketball set-up from the previous activity, “Oval Orbits,” have the students explore what is necessary to achieve a circular or near circular orbit with the weight.
7. Have them tap the weight into an elliptical orbit and tap again at right angles to the ball.
8. Try other angles and observe the results.

Extension:

Investigate orbital and escape velocities. Have students note how much inertia an object must have to achieve orbit. How fast must it go to break away from orbit all together?

demonstration illustration

THE FOLLOWING DEMONSTRATION SHOULD ONLY BE DONE IN A LARGE AREA SUCH AS A PLAYGROUND.

Have the students tie a 2 meter long thread around a mushball or other heavy soft ball. Use light thread as you want it to break. Have a student stand well away from any other students or buildings and swing the ball around him/her to achieve orbit. Have the student swing faster until the ball (spacecraft) breaks the string (gravity.) Observe the path the ball takes once it has broken “the pull of gravity.”

Remember, keep the students back far enough so that the ball hits the ground first before it can be caught.

How is the ball and string like a spacecraft and gravity? What was necessary to “break” the pull of gravity?

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