passage: Speed is an important aspect of motion .
  It is a measure of how fast or slow something moves .
  To determine speed you must know two things .
  First , you must know how far something travels .
  Second , you need to know how long it takes to travel that far .
  Speed can be calculated using this formula : speed = distance time A familiar example is the speed of a car .
  In the U.S. , this is usually expressed in miles per hour .
  Think about a trip you and your family made in the car .
  Maybe the trip covered 120 miles and it took 3 hours .
  What was the cars speed ?
  speed = 120 mi = 40 mi / h 3h The speed of a car may also be expressed in kilometers per hour ( km / h ) .
  The SI unit for speed is meters per second ( m / s ) .
  This is the unit of measure a scientist would use .
query_and_answer: What is speed ? || The measure of gravity against time
evidences: Speed is an important aspect of motion . It is a measure of how fast or slow something moves .