Primary School Space Science for pupils 9 + : smedley definitions

Relativity

Things going in the same direction at the same speed are said to be in an "inertial frame." Because they are moving together at the same speed and distance, they do not look like they are moving to those that are inside that frame. It is similar to people traveling together in a car. The car is traveling in one direction at a certain speed (let's say 50 mph), but the passengers, who are in the same "inertial frame," feel like they are sitting still. However, to an observer standing outside the car, the passengers are moving at 50 mph: the same speed that the car is moving, relative (compared) to the person who is standing still. If someone in the front seat were to toss a snack to the back seat, the movement would look different to the passengers than it would to the observer who is standing outside the car. The passengers in the back seat would see the snack flying backward toward them at approximately 40 mph since they are in the same "inertial frame" as the car. The person standing outside of the car would see the snack fly at approximately 10 mph in the same direction in which the car is traveling (50 mph - 40 mph in the opposite direction). People in the same "frame" measure how long something takes to happen. Their clocks keep the same time. But in another "frame," their clocks move at a different rate.

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https://jwst.nasa.gov/content/about/orbit.html

https://www.quora.com/When-I-use-a-very-sharp-knife-to-cut-an-object-am-I-slicing-through-atoms-or-just-pushing-them-aside

mass matter

https://flexbooks.ck12.org/cbook/ck-12-middle-school-physical-science-flexbook-2.0/section/2.1/primary/lesson/matter-mass-and-volume-ms-ps/

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https://thesciencespace.quora.com/?__ni__=0&__nsrc__=4&__snid3__=14335360809&__tiids__=15048681

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