Orbital Motion

Key points

Two objects with a great difference in mass orbit each other due to the gravitational force and acceleration that they exert upon each other.
- The smaller object orbits the larger.
- Gravitational force provides centripetal force for orbital motion.
Kepler’s Third Law dictates the period of planets’ orbits around the sun.

Orbital Motion is a type of circular motion that two bodies of significantly different masses engage in.

Smaller planets orbit larger stars, and moons orbit planets.
To maintain a constant orbit, frictional forces are at a minimum compared to the gravitational force.
- The gravitational force creates a centripetal force for the circular motion of the orbit.

Not in Formula Booklet but important

For this section, there are two relevant equations, neither of which are included in the formula booklet:

The first equation is used to find orbital velocity.
- Found by setting the expression for centripetal force equal to the expression for gravitational force, then simplifying.
The second equation is Kepler’s Third Law.
- Derived by taking the square of the linear velocity expression (in circular motion), and setting that equal to the square of the velocity, as defined by the first expression.
- This law shows that the period of planets orbiting around a star is proportional to 3/2 of the orbital radius.

This is a summary of the concepts and equations above.

Not in Formula Booklet but important

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