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.

Multiple-Choice Questions

Answers

Free-Response Questions

Circular motion

• 2024 Q1 - - Block sliding on a track with loops, forces, FBDs, circular motion, energy
• 2023 Q3 - - Block and spring rotating about axle, circular motion, centripetal force, FBDs
• 2018 Q1 - - Spacecraft in circular orbit, circular motion, forces, FBDs, law of gravitation

Law of gravitation

• 2024 Q4 - - Pendulum on different planets, law of gravitation, simple harmonic motion, work, spring force
• 2022 Q2 - - Gravitational force between planet and moons, law of gravitation, FBD's
• 2018 Q1 - - Spacecraft in circular orbit, circular motion, forces, FBDs, law of gravitation

• 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

• 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.