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velocity sufficient to cause a natural or artificial satellite to remain in orbit. Inertia of the moving body tends to make it move on in a straight line, while gravitational force tends to pull it down. The orbital path, elliptical or circular, thus represents a balance between gravity and inertia. A cannon fired from a mountaintop will throw a projectile farther if its muzzle velocity is increased. If velocity is made high enough the projectile never falls to the ground. The surface of the Earth may be thought of as curving away from the projectile, or satellite, as fast as the latter falls toward it. The more massive the body at the centre of attraction, the higher is the orbital velocity for a particular altitude or distance. Near the surface of the Earth, if air resistance could be disregarded, orbital velocity would be about eight kilometres (five miles) per second. The farther from the centre of attraction a satellite is, the weaker the gravitational force and the less velocity it needs to remain in orbit. See also escape velocity.