Student text Pages 64-67

ROCKETS AND SATELLITES

TOPIC: ROCKETS AND SATELLITES How does a rocket lift off the ground?

The awesome achievement of lifting a huge rocket into space against the force of gravity can be explained using Newton’s third law of motion.

A rocket can rise into the air because the gases it expels with a downward action force exert an equal but opposite reaction force on the rocket.

As long as this upward pushing force, called thrust, is greater than the downward pull of gravity, there is a net force in the upward direction.

As a result, the rocket accelerates upward.

• What keeps a satellite in orbit?A satellite is any object that travels

around another object in space.Artificial satellites are launched into

orbit around Earth. They aid in space research,

communications, military intelligence, weather analysis, and geographical surveys.

Artificial satellites travel around Earth in an almost circular path. Any force that causes an object to move in a

circular path is called a centripetal force. The word centripetal means “center-

seeking.” For a satellite that orbits Earth, the

centripetal force is the gravitational force that pulls the satellite toward the center of Earth.

Satellites in orbit around Earth continually fall toward Earth, but because Earth is curved, they travel around it.

The faster an object is thrown, the farther it travels before hitting the ground. If thrown fast enough, an object will go around Earth.

Although a satellite in orbit falls because of gravity, Earth’s surface curves away from the satellite at the same rate.

The speed with which an object must be thrown in order to orbit Earth is about 7,900 m/s.

Satellites are placed into orbits that are varying distances from the center of Earth.

Satellites in higher orbits travel more slowly and take longer to circle Earth than do satellites in lower orbits.

For example, a satellite in low orbit might take less than 2 hours to circle Earth.

A satellite orbiting about 36,000 kilometers above the surface of Earth takes about 24 hours to circle Earth.

Since Earth rotates once every 24 hours, a satellite above the equator always stays above the same point on Earth as it orbits.

HOMEWORK

• Chapter 2 Section 5 Assessment

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