That would be velocity.
life goes by too fast
yes you can fast something rather then food you could fast water, TV, facebook, electricity, your phone etc.
When something is said to be very fast, it means that it is quick or rapid or swift. Something that is said to be very fast is time passing when you are enjoying your self. If "Very fast" is said by a timekeeper at the end of a race I'd just run, I'd say "How fast?"
You cling to something when you hold fast to it. You also clench your teeth when you hold them together.
A cheetah
speed
another word for fast is speed
"Fast" is an adjective. It describes the speed at which something moves or happens.
Do something that is fun to you.
how fast something can change
Friction
Something that is hard and fast is a permanent, lasting thing -- "fast" in this sense means fastened down.