There are a few common ways of understanding orbits.
* As the object moves sideways, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
* A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
* As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center.
As an illustration of an orbit around a planet, the much-used cannon model may prove useful (see image below). Imagine a cannon sitting on top of a tall mountain, which fires a cannonball horizontally. The mountain needs to be very tall, so that the cannon will be above the Earth's atmosphere and we can ignore the effects of air friction on the cannon ball.
If the cannon fires its ball with a low initial velocity, the trajectory of the ball curves downwards and hits the ground (A). As the firing velocity is increased, the cannonball hits the ground farther (B) away from the cannon, because while the ball is still falling towards the ground, the ground is increasingly curving away from it (see first point, above). All these motions are actually "orbits" in a technical sense — they are describing a portion of an elliptical path around the center of gravity — but the orbits are of course interrupted by striking the earth.
If the cannonball is fired with sufficient velocity, the ground curves away from the ball at least as much as the ball falls — so the ball never strikes the ground. It is now in what could be called a non-interrupted, or circumnavigating, orbit. For any specific combination of height above the center of gravity, and mass of the object being fired, there is one specific firing velocity that produces a circular orbit, as shown in (C).
As the firing velocity is increased beyond this, a range of elliptical orbits are produced; one is shown in (D). If the initial firing is above the surface of the earth as shown, there will also be elliptical orbits at slower velocities; these will come closest to the earth opposite the firing the point.
At a faster velocity called escape velocity, again dependent on the firing height and mass of the object, an infinite orbit such as (E) is produced — first a range of parabolic orbits, and at even faster velocities a range of hyperbolic orbits. In a practical sense, both of these infinite orbit types mean the object is "breaking free" of the planet's gravity, and "going off into space".
Understanding orbits
There are a few common ways of understanding orbits.
* As the object moves sideways, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
* A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
* As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center.
As an illustration of an orbit around a planet, the much-used cannon model may prove useful (see image below). Imagine a cannon sitting on top of a tall mountain, which fires a cannonball horizontally. The mountain needs to be very tall, so that the cannon will be above the Earth's atmosphere and we can ignore the effects of air friction on the cannon ball.
If the cannon fires its ball with a low initial velocity, the trajectory of the ball curves downwards and hits the ground (A). As the firing velocity is increased, the cannonball hits the ground farther (B) away from the cannon, because while the ball is still falling towards the ground, the ground is increasingly curving away from it (see first point, above). All these motions are actually "orbits" in a technical sense — they are describing a portion of an elliptical path around the center of gravity — but the orbits are of course interrupted by striking the earth.
If the cannonball is fired with sufficient velocity, the ground curves away from the ball at least as much as the ball falls — so the ball never strikes the ground. It is now in what could be called a non-interrupted, or circumnavigating, orbit. For any specific combination of height above the center of gravity, and mass of the object being fired, there is one specific firing velocity that produces a circular orbit, as shown in (C).
As the firing velocity is increased beyond this, a range of elliptical orbits are produced; one is shown in (D). If the initial firing is above the surface of the earth as shown, there will also be elliptical orbits at slower velocities; these will come closest to the earth opposite the firing the point.
At a faster velocity called escape velocity, again dependent on the firing height and mass of the object, an infinite orbit such as (E) is produced — first a range of parabolic orbits, and at even faster velocities a range of hyperbolic orbits. In a practical sense, both of these infinite orbit types mean the object is "breaking free" of the planet's gravity, and "going off into space".