Orbits are the paths followed by objects as they move. Most astronomical objects are in one sort of orbit or another. For example, the Moon orbits the Earth. The Earth orbits the Sun, the Sun orbits the Milky Way, the Milky Way orbits in the Local Group of galaxies and so on.
What determines an object's orbit and what can we learn from it?
An objects' orbit is determined by the forces acting on it. The dominant force is often gravity.
1. What is one property of a body that directly determines its gravitational attraction on another?
To understand how orbital motion operates we need to look more closely at some general principles of motion. These principles are stated in Newton's Laws of Motion..
Newton's first law of motion states
An object at rest remains at rest and an object in motion moves in a straight line at constant speed unless an unbalanced force acts on it.
In the above we have used the term "unbalanced force." By that we mean that the object may have forces acting on it, but if they act in opposite directions with equal force nothing happens. Only if the forces are not equal and opposite will they set an object at rest in motion or deviate a moving object from a straight line. For example, if you push on shopping cart, you set it moving. However, if a friend pushes equally hard on the opposite side of the
cart it doesn't move. In the former case ther force is unbalanced.
In the latter it is balanced.
2. A planet orbiting the Sun follows an approximately circular path. Because the path is not a straight line you can conclude
4. According to Newton's first law, what would happen to the planet in the Figure above if the Force of gravity suddenly shut off?
5. Why doesn't the Sun follow a straight line path as described by Newton's Second Law?
6. What creates the force that holds the Sun in its orbit?