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 their rest masses in (): The energies can be eliminated by: similarly the momenta can be eliminated by: where is the angle between the momentum vectors and . Rearranging: Since the invariant mass of the system and the rest masses of each particle are frame-independent, the right hand side is also an invariant (even though the energies and momenta are all measured in a particular frame). Using the de Broglie relations for energy and momentum for matter waves, where is the angular frequency and is the wavevector with magnitude , equal to the wave number, the energy–momentum relation can be