87. Orbits

An orbit is the curved path of one celestial object or spacecraft around another. This video explains the physics behind why objects orbit and the relationship between orbit size and speed.

1. Why do objects orbit?

Orbits are the result of a balance between two main factors:

• Instantaneous Velocity: The forward speed and direction an object is traveling. According to Newton's First Law, an object would continue in a straight line forever if no other forces acted on it.

• Gravitational Pull: A large mass (like Earth) exerts an attractive force on smaller objects (like the Moon).

Because the orbiting object is moving very fast, gravity isn't strong enough to pull it all the way in. Instead, gravity simply "bends" the object's path into a circle or ellipse.

2. Velocity vs. Acceleration in Orbit

This is a common exam concept that can be confusing:

• Speed: The object's speed remains constant throughout its orbit.

• Velocity: Because the object is constantly changing direction, its velocity is constantly changing (since velocity = speed + direction).

• Acceleration: Because acceleration is defined as a change in velocity, an orbiting object is technically constantly accelerating, even though it isn't getting any faster or slower.

3. Orbit Size and Speed

There is a direct relationship between how far an object is from what it's orbiting and how fast it must travel to stay there:

• Close Orbit: Gravity is much stronger. To avoid being sucked into the larger object, the orbiting body must travel faster.

• Distant Orbit: Gravity is weaker. The orbiting body travels slower to maintain a stable path.

The Rule: The smaller the radius of the orbit, the faster the object must travel to maintain a stable orbit.