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Framerate, or frames per second (FPS), is a measure of how smoothly a graphics display is running.
To simulate motion, many images are shown each second, fooling the brain into thinking that there is a moving object. Film has 30 FPS, which is the passing level for a computer game. Film is actualy more complicated then that, there are different standards which use different frame rates, converting between them results in degraded images
The Simulator has a physics FPS. In an ideal world physics would be calculated continuously and with calculus. Do to computer restrictions this is not possible to do in real-time. So the physics engine devides a second up into frames, each frame it moves all physical objects the distance they would go in that time period and checks to see if as a result of that movement they are currently embedded in another object. If they are, they push away from each other. This solution gives good enough physics in real-time without the need for expensive custom hardware. SL has a hard-coded (or maybe it's in a settings file) max physics FPS of 45.