Students are Expected to Understand

Requirements and Constraints for Maintaining Circular Motion

Perspectives on Centripetal and Centrifugal Forces

Gravity's Effects on Orbits and Motion

Inverse Square Relationships with Regards to Distance and Gravitational Force

Some Notes on Circular Motion

• Inertia does its best to have all objects moving at a constant velocity. This also means these objects keep moving in a straight line. If you want to make an object move in a circle, you have to keep pushing on the object, and you'll have to continuously push in towards the middle of the circle that the object is moving along

• What happens to objects that don't have that push towards the middle any more? Think about what inertia does to all objects without a net force on them

• A centripetal force is whatever force that's pushing towards a middle of a circle that keeps the object moving in a circle

• However, a centrifugal force is a force that seems to push objects away from the middle. Now, if you're inside a turning or spinning object or if you yourself are being spun, you're definitely feeling it. Think of what it's like to be inside a car that's making a hard turn. You can feel yourself getting pushed towards one of the doors! But if you're looking from the outside in, you would see that you're really just trying to move straight like inertia wants you to, and that straight line happens to be up against the car doors. So depending on how you see things, they're both different and right!

• No orbits are truly circular. In fact, they're all elliptical. Think of it as a type of oval or kind of like the shape of an egg (side note: the word oval comes from the earlier word for "egg")

• The closer an orbiting object is to whatever it is orbiting, the faster it will move. This is because gravity increases significantly with shorter distances and is explained by Newton's Law of Universal Gravitation which helps us understand the inverse square rule with distance and force

• When the International Space Station is passing above us, we're closer to those astronauts than we are to Los Angeles. Astronauts in the ISS experience 90% of Earth's Gravity, so how can they be considered weightless? It's not just about where they are, it's also about how they're moving. They're in orbit meaning that they're being thrown sideways instead of being thrown downward. As long as the space station they're in is also moving sideways with the astronauts, they'll never feel like they're being pushed into the floor. They feel like they're floating! This is also why it can be referred to as microgravity even though there's still plenty of it up there