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 object in the lab frame with magnitude . The relativistic energy and momentum include the Lorentz factor defined by: Some authors use relativistic mass defined by: although rest mass has a more fundamental significance, and will be used primarily over relativistic mass in this article. Squaring the 3-momentum gives: then solving for and substituting into the Lorentz factor obtains its alternative form in terms of 3-momentum and mass, rather than 3-velocity: Inserting this form of the Lorentz factor into the energy equation: followed by more rearrangement yields (). The elimination of the Lorentz factor also eliminates implicit velocity dependence of