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
Energy–momentum relation is consistent with this equation – it is consistent with relativistic mechanics, and is Lorentz invariant. In relativistic quantum field theory, it is applicable to all particles and fields. The Energy–momentum relation was first established by Paul Dirac in 1928 under the form formula_3, where V is the amount of potential energy. The equation can be derived in a number of ways, two of the simplest include: For a massive object moving at three-velocity with magnitude in the lab frame: is the total energy of the moving object in the lab frame, is the three dimensional relativistic momentum of the