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Displacement and Distance in 2 and 3 Dimensions

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3.4Displacement and Distance in 2 and 3 Dimensions


In these revision notes for Displacement and Distance in 2 and 3 Dimensions, we cover the following key points:

  • The difference between Displacement and Distance in two dimensions
  • How to calculate the Displacement and Distance in two dimensions
  • The same for the three dimensional motion

Displacement and Distance in 2 and 3 Dimensions Revision Notes

Displacement represents the shortest path between two points in a line, plane or space while Distance is the length of the true path followed by the object during its motion. Displacement can be shorter or equal to the Distance but it cannot be longer.

In two dimensions, if the initial position of the moving object is the vector r1 and the its final position is the vector r2, we can write for the displacement ∆r

∆r = r2 - r1

Splitting the vectors in components, we obtain

∆rx = r2x - r1x

and

∆ry = r2y - r1y

Therefore, the magnitude of the Displacement vector ∆r is

|∆r| = √∆r2x + ∆r2y
= √(r2x - r1x)2 + (r2y - r1y)2

When an object is moving in space, we call it "3D motion." In this case, an additional coordinate (usually denoted by z) is required to represent the third dimension.

However, the approach is the same as for 2D motion. We simply write the z-coordinate besides the x- and y-coordinates and all calculations rely on these three coordinates.

Thus, if the vector ri shows the initial position and the vector rf the final position of the object in space, both them have three coordinates each: xi, yi and zi for ri and xf, yf and zf for rf. Therefore, the displacement vector

∆r = rf - ri

will have three coordinates as well. They are:

∆rx = rfx - rix
= xf-xi
∆ry = rfy - riy
= yf - yi
∆rz = rfz - riz
= zf - zi

The magnitude of the Displacement vector ∆r therefore is

|∆r| = √∆r2x + ∆r2y + ∆r2z

Or

|∆r| = √(xf - xi)2 +(yf - yi)2 + (zf - zi)2

Conceptually, there is nothing new here; the only difference with the study of 2-D motion is the new coordinate z, which makes the equations longer, but the structure is the same.

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