Simplify the following text for a Grade 9 native English-speaking person.
Original: The energy E of movement is assumed to be small compared to their mass , allowing one to describe the bodies with non-relativistic mechanics .
Simplified: Scientists use a short-cut when they talk about this energy , they call it ` E. ' The energy E of movement is assumed by scientists to be small compared to their mass , allowing such experts to describe the bodies with non-relativistic mechanics .