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Problems and Solutions about distance and displacement are presented and updated useful for highschool and college students.

Problem 1: 

An object moves from point A to B, C, and D finally, along a rectangle. 
(a) Find the magnitude and direction of the displacement vector of the object?
(b) Find the distance traveled by that object?
(c) Suppose the object returns to the point A, its initial position. Now, Find the displacement and distance?

 

displacement example around rectangle
Solution to problem 1:
(a) By definition of displacement, connect the initial (A) and final (D) points together. As shown, displacement is toward the negative of the y-axis and its magnitude is equal to the width of the rectangle. Displacement is used for solving speed and velocity problems

 

displacement around rectangle

(b) distance is equal to the length of AB+BC+CD.
(c) Since the initial and final positions are the same, so by definition of displacement, the difference between them is zero. But distance, in this case, is the perimeter of the rectangle.


Problem 2:

An object moves along a right triangle from point A to B to C shown in the figure below. (Consider the sides as $3\,{\rm m}$ and $4\,{\rm m}$)
(a) Find the magnitude and direction of the displacement vector?
(b) How much distance traveled by this moving object?
(c) Suppose the object returns to the point A, its initial position. Now, Find the displacement and distance?

 

displacement around triangle

Solution to problem 2:
(a) Connect the initial (A) and final (C) points together by the straightest line. The magnitude and direction of that line represent the displacement vector. Here, the displacement is the hypotenuse of the right triangle. So using the Pythagorean theorem, one finds its magnitude as
\begin{align*}
c^2 &= a^{2} + b^{2} \\
\Rightarrow c&=\sqrt{a^{2} +b^{2}}\\
&= \sqrt{3^{2} + 4^{2}}=5\,{\rm m} 
\end{align*}
the direction of displacement is shown in figure (to the northeast).

displacement around right triangle is toward its hypotenuse.

(b) Adding the base and height of this triangle gets the distance traveled from (A) to (C).
\begin{align*}
\text{distance} &= a + b\\
                                &= 3 + 4\\
                                &= 7\,{\rm m}
\end{align*}

(c) Displacement is zero since the object returns to the initial position. Distance is also computed as the perimeter of the triangle.


Problem 3: 
A displacement vector has a magnitude of $810\,{\rm m}$, and points at an angle of $18^\circ$ above the positive x-axis. What are the $x$ and $y$ components of this vector?

Solution to problem 3:
Given the length and direction of a vector such as displacement $\vec{R}$ , one can find its components by the following formulas
\begin{align*}
R_x &= |\vec{R}|\cos \theta \\
&=(810)\cos 18^\circ=770.35\,{\rm m}\\
\text{And}\\
R_y&=|\vec{R}|\sin \theta \\
&=(810)\sin 18^\circ=250.30\,{\rm m}
\end{align*}

Decomposition of a vector

Problem 4:
A displacement vector is $23\,{\rm km}$ in length and directed $65^\circ$ south of east. What are the components of this vector?

Solution to problem 4:
Like previous problem, displacement in the components form is as $\vec{R}=|\vec{R}|\,\cos \theta \hat{i}+|\vec{R}|\,\sin \theta \hat{j}$ where $\theta$ is measured with the positive $x$-axis. Here, the given angle is below the $x$-axis so we have
\begin{align*}
R_x &= |\vec{R}|\cos \theta \\
&=(23)\cos (-65^\circ)=9.72\,{\rm m}\\
\text{And}\\
R_y&=|\vec{R}|\sin \theta \\
&=(23)\sin (-65^\circ)=-20.84\,{\rm m}
\end{align*}
The minus sign of the y-component indicates the south direction.


Problem 5:
A race car will make one lap around a circular track of radius $R$. When the car has traveled halfway around the track, what is the magnitude of displacement of the car from the starting point? Find the distance of the car?

Solution to problem 5:
Displacement is the difference between initial and final points so in this case, that difference is the diameter of the circular track. Thus,
\[\text{displacement}=2R\]
Distance is also the circumference of the semi-circle that is $2\,\pi R$.


Problem 6:
You walk $5\,{\rm km}$ due south then $4\,{\rm km}$ due west. What is the magnitude of your displacement?

Solution to problem 6:
$5\,{\rm km}$ south means a position vector of magnitude $5\,{\rm km}$ from the origin toward the south. The other journey starts from the tail of the previous vector with magnitude and direction of $4\,{\rm km}$ and west, respectively. 

In the mathematical language the above two vectors are written as $\vec{R}_1=5\,(-\hat{j})$ and $\vec{R}_2=4\,(-\hat{i})$. To find the displacement we have two options, either using the drawing vector or the algebraic method which is efficient for more involved problems.

Method I: 
The vector that constructed from connecting the tip of the first vector to the tail of the second vector is displacement (triangle method). Use the Pythagorean theorem to get the magnitude of the desired vector.

Adding two consecutive vectors lead to the displacement
Method II:
In the case of several journey segments, displacement can be determined using the methods of vector addition. Here, the whole journey is formed of the two segments with the corresponding vectors $\vec{R}_1=5\,(-\hat{j})$ and $\vec{R}_2=4\,(-\hat{i})$. Thus, using the vector addition method we get 
\begin{align*}
\vec{R}&=\vec{R}_1+\vec{R}_2\\
&=5\,(-\hat{j})+4\,(-\hat{i})\\
\text{or} \ &=-4\,\hat{i}-5\,\hat{j}
\end{align*}
The magnitude and direction of the above vector is determined by the following formulas
\begin{align*}
|\vec{R}|&=\sqrt{R_1^2 + R_2^2}\\
&=\sqrt{(-5)^2 + (-4)^2}\\
&=\sqrt{41}\,{\rm km}\\
\text{And}\\
\theta &=\tan^{-1}\left(\frac{R_y}{R_x}\right)\\
&=\tan^{-1}\left(\frac{-5}{-4}\right)\\
&=51.34^\circ
\end{align*}
Since the $x$ and $y$ components of the displacement place in the third quadrant so the above angle is toward the west south. 
 

Important notes about displacement and distance:

  • If the initial and final points are the same, so the total displacement is zero.
  • Distance is measured as the perimeter of the path traveled by the moving object.

For more involved and difficult problems with a thorough explanation about the definition of displacement in two and three dimensions visit the exam center or course pages.

In addition, you can also check out the Wikipedia article about distance and displacement.

Last Updated : 7/31/2020