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Activity 4
Around The World
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OBJECTIVE:
To model how satellites orbit Earth.
BACKGROUND:
The manner in which satellites orbit Earth is often explained as a balance
that is achieved when the outward-pulling centrifugal force of a revolving
object is equal to the inward pull of gravity. However, if we examine
Isaac Newton's First Law of Motion, we can see why this explanation is
incomplete. According to this law, objects in motion remain in motion
in a straight line unless acted upon by an unbalanced force. Because Earth-orbiting
objects follow elliptical paths around Earth and not a straight line,
forces cannot, by definition, be balanced. Force is directional. It is
a push or a pull in a particular direction. At any one moment, the force
of gravity on a satellite is exerted in the direction of a line connecting
the center of mass of Earth to the center of mass of the satellite. Because
the satellite is not stationary, the direction of this line, and consequently
the direction of the force, is constantly changing. This is the unbalanced
force that curves the path of the satellite.
A second problem with the satellite orbit explanation is that centrifugal
force is not an actual force but an effect. The difference is important.
For example, if you are a passenger riding in a car that makes a sharp
turn to the left, you feel yourself pushed against the right side
door. This is interpreted as an outward directed force but is it really
an outward directed force? What would happen to you if the door were
to open suddenly? Rather than try to answer these questions in an
automobile, a simple demonstration can be done. Attach a ball to a
string and twirl the ball in a circle as you hold the other end of
the string. The ball travels on a path similar to a satellite orbit.
Feel the outward pulling force as you twirl the ball. Next, release
the ball and observe where it goes. If that force you experienced
were really outward, the ball would fly straight away from you. Instead,
the ball travels on a tangent to the circle.
What is actually happening is that the ball is attempting to travel
in a straight line due to its inertia. The string, acts as an unbalanced
force that changes the ball's path from a straight line to a circle.
The outward pull you feel is really the ball's resistance to a change
in direction. Through the string, you are forcing the ball from
a straight path to a circle. In the case of the automobile example,
if the door were to pop open during a turn, you would fall out of
the car and continue moving in the same direction the car was moving
at the moment the door opened. While you perceive your motion as
outward, the automobile is actually turning away from you as you
go in a straight line.
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In this demonstration, a simple model of a satellite orbiting
Earth is created from a large stationary ball and a smaller ball
at the end of a string. The ball and string become a pendulum that
tries to swing toward the middle of the globe. However, the ball
travels in an orbit around the globe when it is given a horizontal
velocity in the correct direction. Although the small ball attempts
to fall to the center of the larger ball, its falling path becomes
circular because of its horizontal velocity.
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MATERIALS NEEDED:
Large ball
Small ball
2 meters of string
Flower pot
PROCEDURE:
Step 1. Attach the 2 meter long string to the smaller ball
(satellite). This can be done by drilling or poking a hole through
the ball, threading it through to the other side, and knotting the
string.
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Step 2. Place the large ball
(Earth) on the flower pot in the center of an open space.
Step 3. Select one student to stand above Earth and hold the
satellite by the end of the string attached to it. This student's
hand should be directly over the "north pole" of Earth and the satellite
ball should rest against the side of Earth at its "equator."
Step 4. Select a second student to launch the satellite. Try
pushing the satellite straight out from Earth. Try launching the satellite
in other directions. |
QUESTIONS:
- What path does the satellite follow when it is launched straight out
from Earth?
- What path does the satellite follow when it is launched at different
angles from Earth's surface?
- What affect is there from launching the satellite at different speeds?
- Is it correct to say that a satellite is in a continual state of free-fall?
Why doesn't the satellite strike Earth?
- What causes a satellite to return to Earth?
FOR FURTHER RESEARCH:
- Investigate the mathematical equations that govern satellite orbits
such as the relationship between orbital velocity and orbital radius.
- Learn about different kinds of satellite orbits (e.g., polar, geostationary,
geosynchronous, etc.) and what they are used for.
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