3.1 - Position and displacement

This section looks at how physics describes and quantifies object locations. By the end of this section you should be able to apply the concepts of position, coordinates, vectors, and displacements to scenarios involving motion. Read the text aloud
Distance and position
Space has dimensions of length, and distance is the separation between any two points in space. Saying an object is 3 m away does not tell you where the object is located, however, because distance does not include any information about the starting position or the direction of the motion. Read the text aloud


Origin of a one-dimensional system
Position in physics means a unique location in space with reference to an origin and a coordinate system. Position has units of length but may be positive or negative depending on direction. For example, a meter stick defines a one-dimensional straight line with the origin at the zero mark. In this example positive positions are to the right of the origin and negative positions are to the left. A position of −0.50 m is a half meter to the left of the start of the meter stick. The position of −0.5 meters is different from a distance of 0.5 m because position implies an origin. Read the text aloud Show Three-dimensional space
Origin of the geographical coordinate system of latitude and longitudeThe origin is a fixed reference point that you choose. In laboratory experiments you might choose the origin to be a particular point on the table or the top of a ramp. The key idea is that all position measurements are given relative to the same origin point; otherwise you would not know how to interpret them correctly. GPS devices use latitude and longitude to specify position on Earth’s surface relative to zero longitude at the prime meridian in Greenwich, England. Read the text aloud
Displacement and positionA displacement is a change in position. Displacement can have positive and negative values, as can position. When negative is defined to be left of the origin, a negative displacement is to the left. A positive displacement is to the right. The position of an object is equal to its starting position plus any displacement. Read the text aloud
Describe an action that will take you from your seat to the classroom door without using the word “displacement.” What other words or phrases mean the same thing? For example, you might say, “I should _________ about three meters to my left and two meters straight ahead.” You don’t need exact distances, just an estimate. Read the text aloud
Test your knowledge:  displacement and positionA ball undergoes a displacement of −20 m from its initial position shown in the diagram. Which of the following is true?
  1. The ball moves a total distance of −30 m.
  2. The ball moves a total distance of 30 m.
  3. The ball ends up at a position of −30 m.
  4. The ball ends up at a position of +10 m.
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