| Gravitational Source: Click anywhere in the simulation window to place a gravitation source. A picture of the source (a planet) appears in the lower right corner. You cannot place a gravitation source if any horizontal speeds or forces are present. The default gravitation source is Earth. In the source's Properties Box, you can change the gravitation source to the Moon, Jupiter, or Planet X. If you choose Planet X, use the sliding scale labeled Gravity Rate to determine the strength of the gravitational acceleration. A Gravity Rate of 1.0 is equal to Earth gravity. Bodies experience gravity as a constant acceleration in the -Y direction. When gravity is present, all bodies have gravity force vectors attached that are labeled "Gravity force." Bodies are constrained to move in the vertical direction. Bodies constrained to move on horizontal surfaces also experience normal and possibly friction forces. In the Properties Box of the gravity force vector, you can opt to show or not show the gravity force label, or show (or hide) the gravity force during Stop mode, Run mode, or Pause mode.