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
Gravitational energy get the total work done by an external force to bring point mass formula_3 from infinity to the final distance formula_10 (for example the radius of Earth) of the two mass points, the force is integrated with respect to displacement: Because formula_13, the total work done on the object can be written as: In general relativity gravitational energy is extremely complex, and there is no single agreed upon definition of the concept. It is sometimes modeled via the Landau–Lifshitz pseudotensor that allows retention for the energy-momentum conservation laws of classical mechanics. Addition of the matter stress–energy–momentum tensor to the Landau–Lifshitz