Eighth Grade · Science · week 1

Measure motion: speed graphs

Calculate speed from distance and time data, and read a distance-time graph to compare how fast objects move.

Speed tells you how fast position is changing: how much distance something covers in a given amount of time. The formula is speed = distance / time. A speed of 5 meters per second (m/s) means that for every second that passes, the object covers another 5 meters.

A distance-time graph turns motion into a picture. Time runs along the bottom, distance runs up the side, and the steepness of the line -- its slope -- is the speed. A steep line means distance is piling up fast; a flat line means the object has stopped. Whichever line is steeper belongs to the faster object.

📋 For the teacher
Teach it (10–15 min):
  • Work the formula both ways: given distance and time find speed; given speed and time find distance.
  • Graph a real trip together (a car ride, a walk) with two or three data points -- their own data becoming a line makes slope-as-speed click.
  • When comparing two lines, point at the steepness directly before ever computing a number.
  • Say the units out loud with every number ("20 meters divided by 4 seconds") so the answer's unit (m/s) is never a mystery.
Talk about it:
  • Why does a steeper line on a distance-time graph mean a faster speed?
  • What would a distance-time graph look like for something that is not moving?

Mastery looks like: They compute speed from distance/time data and identify the faster object from the steepness of two lines on a graph.

Common stumbles: Dividing time by distance instead of distance by time; assuming a steeper line always means "more distance" rather than "faster speed."

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