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 relates total energy to the (total) relativistic mass (alternatively denoted or ), while relates rest energy to (invariant) rest mass . Unlike either of those equations, the energy–momentum equation () relates the "total" energy to the "rest" mass . All three equations hold true simultaneously. A more general form of relation () holds for general relativity. The invariant mass (or rest mass) is an invariant for all frames of reference (hence the name), not just in inertial frames in flat spacetime, but also accelerated frames traveling through curved spacetime (see below). However the total energy of the particle and its