Gravity: Why Satellites Stay in Orbit

Use direction arrows to explain circular motion and apparent weightlessness.

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For an ideal circular orbit, velocity is tangent to the path and gravitational acceleration is inward. Speed can stay constant while velocity changes direction. Occupants float because they and their spacecraft are in free fall together.

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A satellite shown to the right of Earth moves upward along the circle at that instant. Its gravitational acceleration points left, toward Earth, not upward along its velocity arrow.

A circular orbit around Earth has a spacecraft at the right. Velocity points upward tangent to the path; gravitational acceleration points left toward Earth's center. The drawing is not to scale.

Would you believe it?

Earth is in orbit too: it moves around the Sun under gravity. An orbit combines motion with a continually changing direction.

NASA: gravity and mechanics

Question 1 At the shown position, which direction is gravitational acceleration? Toward Earth's center Along the forward velocity arrow Away from Earth

Question 2 Can velocity change when speed is constant? No, speed and velocity are identical Only when mass changes Yes, its direction can change

Question 3 Why do astronauts float relative to an orbiting cabin? There is no gravity at their altitude They and the cabin fall together They have no mass

Question 4 Without the inward force, the ideal satellite initially follows what path? A straight tangent A straight line toward the center The same circle forever

Question 5 What limitation belongs to this picture? It proves every orbit is a perfect circle It shows gravity ends at the circle It uses a circular, simplified orbit

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