We learned that all objects attract each other with a gravitational force. The earth and the moon attract each other, the earth and the sun attract each other, and so on. If that's the case, why doesn't the moon crash into the earth? Is there another force pushing the moon away from the earth?

Now that we've learned about gravity, uniform circular motion and centripetal force, let's take a look at something that combines these concepts: orbital motion.

An orbit is the circular or elliptical path of an object as it moves around another object, with only the force of gravity acting on it. The moon and thousands of satellites are in orbit around the earth, and the earth and the other planets are in orbit around the sun. Since gravity is the only force involved, we'll see how objects in orbit are really in projectile motion or free fall and have zero apparent weight.

If an orbit is a circle, gravity is the centripetal force causing the circular motion. That means we can combine the equations for gravitational force and centripetal force to find the object's orbital radius, speed and period.

We'll also take a look at Kepler's Laws of planetary motion. With the help of others, Johannes Kepler discovered that the orbit of a planet around the sun actually follows an ellipse instead of a circle. These laws can tell us even more about orbital motion.

Multiple-Choice Questions

Answers

- Orbits and Centripetal Force

• Khan Academy - Gravity for astronauts in orbit
• Matt Anderson - Hit a golf ball into orbit
• Michel van Biezen - Acceleration and Weight of a Satellite in Orbit
• Michel van Biezen - Orbital Velocity

- Kepler's Laws

• Matt Anderson - Kepler's Laws
• Professor Dave - Kepler's Laws and Beyond

• Khan Academy - Acceleration due to gravity at the space station
• Khan Academy - Space station speed in orbit
• Organic Chemistry Tutor - Speed of a Satellite in Circular Orbit
• Organic Chemistry Tutor - Kepler's Third Law of Planetary Motion

• Smarter Every Day - How to Fly a Spaceship to the Space Station
• Space Flight: The Application of Orbital Mechanics

• Orbits
• Elliptical Orbits & Kepler's 2nd Law
• Two-body problem simulator

Other Resources

We learned that all objects attract each other with a gravitational force. The earth and the moon attract each other, the earth and the sun attract each other, and so on. If that's the case, why doesn't the moon crash into the earth? Is there another force pushing the moon away from the earth?

Now that we've learned about gravity, uniform circular motion and centripetal force, let's take a look at something that combines these concepts: orbital motion.

An orbit is the circular or elliptical path of an object as it moves around another object, with only the force of gravity acting on it. The moon and thousands of satellites are in orbit around the earth, and the earth and the other planets are in orbit around the sun. Since gravity is the only force involved, we'll see how objects in orbit are really in projectile motion or free fall and have zero apparent weight.

If an orbit is a circle, gravity is the centripetal force causing the circular motion. That means we can combine the equations for gravitational force and centripetal force to find the object's orbital radius, speed and period.

We'll also take a look at Kepler's Laws of planetary motion. With the help of others, Johannes Kepler discovered that the orbit of a planet around the sun actually follows an ellipse instead of a circle. These laws can tell us even more about orbital motion.