It is important to note that the distance traveled, however, can be greater than the magnitude of the displacement (by magnitude, we mean just the size of the displacement without regard to its direction that is, just a number with a unit). The professor’s initial position is x 0 = 1. When you begin a problem, you should select which direction is positive (usually that will be to the right or up, but you are free to select positive as being any direction). In one-dimensional motion, direction can be specified with a plus or minus sign. The professor’s displacement is 2.0 m to the right, and the airline passenger’s displacement is 4.0 m toward the rear. Note that displacement has a direction as well as a magnitude. Notice that the arrow representing his displacement is twice as long as the arrow representing the displacement of the professor (he moves twice as far) in Figure 2.3. 0 -m displacement of the passenger relative to the plane is represented by an arrow toward the rear of the plane. His location relative to the airplane is given by x x. The word “displacement” implies that an object has moved, or has been displaced.įigure 2.4 A passenger moves from his seat to the back of the plane. This change in position is known as displacement. If an object moves relative to a reference frame (for example, if a professor moves to the right relative to a white board or a passenger moves toward the rear of an airplane), then the object’s position changes. To describe the position of a person in an airplane, for example, we use the airplane, not the Earth, as the reference frame. (See Figure 2.3.) In other cases, we use reference frames that are not stationary but are in motion relative to the Earth.
For example, a rocket launch would be described in terms of the position of the rocket with respect to the Earth as a whole, while a professor’s position could be described in terms of where she is in relation to the nearby white board. Earth is often used as a reference frame, and we often describe the position of an object as it relates to stationary objects in that reference frame. More precisely, you need to specify its position relative to a convenient reference frame. In order to describe the motion of an object, you must first be able to describe its position-where it is at any particular time. (credit: Suzan Black, Fotopedia) Position Their motion can be described by their change in position, or displacement, in the frame of reference. Figure 2.2 These cyclists in Vietnam can be described by their position relative to buildings and a canal.
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