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
Tests of relativistic energy and momentum overview. In classical mechanics, kinetic energy and momentum are expressed as On the other hand, special relativity predicts that the speed of light is constant in all inertial frames of references. The relativistic energy–momentum relation reads: from which the relations for rest energy formula_3, relativistic energy (rest + kinetic) formula_4, kinetic energy formula_5, and momentum formula_6 of massive particles follow: where formula_8. So relativistic energy and momentum significantly increase with speed, thus the speed of light cannot be reached by massive particles. In some relativity textbooks, the so-called "relativistic mass" formula_9 is used as well. However, this concept is considered