2.1 - Motion

Nothing in nature stays in the same place. Things are always moving. One of the earliest goals of physics was to understand motion. This section looks at how we describe quantitatively how objects change their position over time. You already have an intuitive sense of speed and position from experience with bicycles, cars, and maps. Physics will expand the intuitive understanding you already have and make it mathematically rigorous. By the end of this section you should be able to apply the concepts of position, speed, and acceleration to solve motion problems.
Distance and position
The first step in describing how things move is to create a unique way to identify where things are, meaning their precise location in space. Space has units of length and distance is the separation between any two points in space. Saying an object is 3 meters away does not, however, tell you where the object is located, because distance does not include any information about direction or from where the distance is measured.
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 also includes direction information (+ or −) and references a "zero" location, or origin. Consider a meter stick that defines a one-dimensional straight line with the origin at the zero mark. Every location along this line can be identified by a single number. In this diagram, positive positions are to the right of the origin and negative positions are to the left of the origin. A position of −0.50 meters 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 meters because position implies an origin and a coordinate system. Note: the choice of which direction is positive is arbitrary and can change! Show Three-dimensional space
Origin of the geographical coordinate system of latitude and longitude The 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. Global Positioning Satellite (GPS) devices use latitude and longitude to specify position on Earth’s surface. The origin for zero longitude is called the Prime Meridian which passes through the Royal Observatory in Greenwich, England.
Displacement and position A 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 the position it starts plus the displacement it moves.
Test your knowledge:  displacement and position A ball moves a displacement of −20 meters from where it is 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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