What are the two main forces acting on an object in orbit?

In the context of objects in orbit, the two main forces that govern their motion are gravity and inertia.

Gravity is the attractive force between two masses, which pulls the orbiting object towards the larger body it is orbiting around, such as a planet or star. This gravitational pull is what keeps the object in its elliptical or circular path instead of allowing it to fly off into space.

Inertia, on the other hand, is a property of matter that causes an object to remain at rest or to move in a straight line at a constant speed unless acted upon by an external force. An object in orbit is constantly moving forward due to its inertia while simultaneously being pulled towards the larger body by gravity.

The balance between gravity pulling the object inward and inertia keeping it moving outward results in the stable orbit. This interplay of forces explains the circular or elliptical paths that objects take around a larger body, which is fundamental to understanding orbital mechanics.

The other forces mentioned—magnetism, friction, pressure, and density—do not play critical roles in the basic dynamics of orbital motion in the context typically covered in an introductory astronomy course.