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 the particle in (), as well as any inferences to the "relativistic mass" of a massive particle. This approach is not general as massless particles are not considered. Naively setting would mean that and and no energy–momentum relation could be derived, which is not correct. In Minkowski space, energy (divided by ) and momentum are two components of a Minkowski four-vector, namely the four-momentum; (these are the contravariant components). The Minkowski inner product of this vector with itself gives the square of the norm of this vector, it is proportional to the square of the rest mass of the body: