Mechanical motion of bodies. Trajectory
A change of a body’s position relative to other bodies with time is called mechanical motion. Motion is relative: a passenger is at rest relative to the train carriage but moving relative to the ground. The body relative to which motion is considered is the reference body. The line a body traces (or passes along) during motion is its trajectory; if it is a straight line the motion is rectilinear, if a curve it is curvilinear. If the size of a body can be ignored in a given problem, we treat it as a material point (point mass): for example a bus travelling from Tashkent to Samarkand is a dot on a map. Even though the Earth orbits the Sun, its size is tiny compared with the distance to the Sun, so it can be treated as a material point.
“At rest or moving?”: pupils discuss whether a classmate sitting in the classroom is at rest or moving “relative to the desk” and “relative to the Sun”. Safety: when observing traffic, stand only on the pavement and never step into the road.