You walk to the shop and come back to the bank. How far did you walk, and how far are you from home? Two different answers are both right. In this course we choose an axis and a starting point, we separate the path length from the displacement, we find the average and the instantaneous velocity, and we read the story of a motion in a position-time graph.
3 lessons, about 1 h 15 min3 simulations12 problems6 practice questionsPython in every lesson
To describe motion along a line we choose an origin and a positive direction; the position then has a sign. The path length adds up every metre travelled and is never negative. The displacement depends only on the start and the end, can be negative or zero, and is never larger in size than the path length.
Average speed is the path length divided by the time; average velocity is the displacement divided by the time, with a sign. The velocity at an instant is the limit of the average velocity as the time interval shrinks: , the slope of the tangent to the position-time graph. Its size is the speed, which a speedometer shows.
A position-time graph tells the whole story of a motion. A flat line means rest, a straight sloping line a steady velocity (rising for positive, falling for negative), and a curve a changing velocity. The slope at any point is the velocity there; where two bodies' lines cross, they are at the same place at the same time.
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