# Circular Motion

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More Lessons for High School Physics

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A series of free Online High School Physics Video Lessons.

In this lesson, we will learn

• the difference between rotation and revolution
• how to calculate rotational speed
• how to calculate centripetal force
• about centrifugal force and simulated gravity

Rotation and Revolution
Understanding the differences between rotations and revolutions
It is important to understand the difference between rotations and revolutions. When an object turns around an internal axis (like the Earth turns around its axis) it is called a rotation. When an object circles an external axis (like the Earth circles the sun) it is called a revolution

Rotational Speed
The speed at which an object rotates or revolves is called rotational speed. Unlike linear speed, it is defined by how many rotations an object makes in a period of time. The formula for rotational speed is Rotational speed = rotations / time but linear speed = distance / time.
Understanding and calculating rotational speed.

Centripetal Force
Centripetal force causes an object in motion to continue in a curved path rather than a linear one. If this force stops, the object will continue in a tangential linear path. An example of centripetal force is how planets rotate around the sun. In this case gravity is a centripetal force because it keeps the planets on curved paths and we say that centripetal acceleration = velocity2 /radius while centripetal force = mass × velocity2 / radius.
A brief overview of centripetal force
The direction of the force in cases of circular motion at constant speeds

Centrifugal Force
Many students call centripetal force centrifugal force, but the two are not the same. Centrifugal force is the reaction to centripetal force as explained by Newton's Third Law of Motion, but it is only important in its distinction from centripetal force.
Recognizing the differences between centripetal and centrifugal force.

Simulated Gravity
Gravitational force cannot be simulated, but the illusion of gravity can. All it takes for us to think we are being acted upon by gravity, is a force pulling us against a surface, generally the earth. Because centripetal force keeps an object moving in a curved path, it can make you feel like you're being pressed outwards. While this doesn't replicate gravity, it creates a feeling of simulated gravity.
How to simulate gravity using centripetal force.
Space station design and simulated gravity.

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