Toy designed for older audiences, such as teenagers or adults, demonstrate advanced concepts. Newton's cradle, a desk toy designed by Simon Prebble, demonstrates the conservation of momentum and energy. Not all toys are appropriate for all ages of children. Even some toys which are marketed for a specific age range can even harm the development of children in that range. "Age compression" is a toy industry term that describes the modern trend of children moving through play stages faster than they did in the past. Children have a desire to progress to more complex toys at a faster pace, girls in
Relativistic angular momentum of the object's center of mass. Since "T" is the energy density of the object, the spatial coordinates of the center of mass are given by Setting "Y" = X obtains the orbital angular momentum density about the centre-of-mass of the object. The conservation of energy–momentum is given in differential form by the continuity equation where ∂ is the four gradient. (In non-Cartesian coordinates and general relativity this would be replaced by the covariant derivative). The total angular momentum conservation is given by another continuity equation The integral equations use Gauss' theorem in spacetime The torque acting on a point-like