<h2>Kinematics in Two Dimensions: An Introduction</h2>
<div class="wp-caption alignnone" id="attachment_97590" style="width: 800px"><a href="https://data.ulearngo.com/assets/244b33e3-ab05-4293-95e6-dbaf02d77822"><img alt="new-york" class="size-full wp-image-97590" height="600" src="https://data.ulearngo.com/assets/244b33e3-ab05-4293-95e6-dbaf02d77822" width="800"/></a><p class="wp-caption-text">Walkers and drivers in a city like New York are rarely able to travel in straight lines to reach their destinations. Instead, they must follow roads and sidewalks, making two-dimensional, zigzagged paths. Image credit: Margaret W. Carruthers</p></div>
<h2>Two-Dimensional Motion: Walking in a City</h2>
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<p id="import-auto-id1165298535384">Suppose you want to walk from one point to another in a city with uniform square blocks, as pictured in the <a href="#attachment_97591">figure below</a>.</p>
<div class="wp-caption alignnone" id="attachment_97591" style="width: 710px"><a href="https://data.ulearngo.com/assets/4ea42800-845e-40f9-b8a2-00ba4bd65421"><img alt="walking-pedestrian" class="size-full wp-image-97591" height="322" src="https://data.ulearngo.com/assets/4ea42800-845e-40f9-b8a2-00ba4bd65421" width="710"/></a><p class="wp-caption-text">A pedestrian walks a two-dimensional path between two points in a city. In this scene, all blocks are square and are the same size. Image credit: OpenStax, College Physics</p></div>
<p id="import-auto-id1165296716128">The straight-line path that a helicopter might fly is blocked to you as a pedestrian, and so you are forced to take a two-dimensional path, such as the one shown. You walk 14 blocks in all, 9 east followed by 5 north. What is the straight-line distance?</p>
<p id="import-auto-id1165296310636">An old adage states that the shortest distance between two points is a straight line. The two legs of the trip and the straight-line path form a right triangle, and so the Pythagorean theorem, \(a^2 + b^2 = c^2\), can be used to find the straight-line distance.</p>
</section>
<div class="wp-caption alignnone" id="attachment_97592" style="width: 265px"><a href="https://data.ulearngo.com/assets/343e1523-edae-499f-92a4-47c60f36c264"><img alt="pythagorean-theorem" class="size-full wp-image-97592" height="153" src="https://data.ulearngo.com/assets/343e1523-edae-499f-92a4-47c60f36c264" width="265"/></a><p class="wp-caption-text">The Pythagorean theorem relates the length of the legs of a right triangle, labeled \(a\) and \(b\), with the hypotenuse, labeled \(c.\) The relationship is given by: \(a^2 + b^2 = c^2.\) This can be rewritten, solving for \(c : c = \sqrt{a^2 + b^2}.\)</p></div>
<section data-depth="1" id="fs-id1165298595412">
<p id="import-auto-id1165298883906">The hypotenuse of the triangle is the straight-line path, and so in this case its length in units of city blocks is \(\sqrt{(9\text{ blocks})^2 + (5\text{ blocks})^2} = 10.3\text{ blocks},\) considerably shorter than the 14 blocks you walked. (Note that we are using three significant figures in the answer. Although it appears that “9” and “5” have only one significant digit, they are discrete numbers. In this case “9 blocks” is the same as “9.0 or 9.00 blocks.” We have decided to use three significant figures in the answer in order to show the result more precisely.)</p>
<div class="wp-caption alignnone" id="attachment_97593" style="width: 710px"><a href="https://data.ulearngo.com/assets/4a0a8d3b-0d59-4e7a-87f8-f92f01e6fdf7"><img alt="helicopter-path" class="size-full wp-image-97593" height="324" src="https://data.ulearngo.com/assets/4a0a8d3b-0d59-4e7a-87f8-f92f01e6fdf7" width="710"/></a><p class="wp-caption-text">The straight-line path followed by a helicopter between the two points is shorter than the 14 blocks walked by the pedestrian. All blocks are square and the same size. Image credit: OpenStax, College Physics</p></div>
<p id="import-auto-id1165298981862">The fact that the straight-line distance (10.3 blocks) in the <a href="#attachment_97593">figure above</a> is less than the total distance walked (14 blocks) is one example of a general characteristic of vectors. (Recall that <strong><span data-type="term" id="import-auto-id1165298541000">vectors</span></strong> are quantities that have both magnitude and direction.)</p>
<p id="import-auto-id1165298648314">As for one-dimensional kinematics, we use arrows to represent vectors. The length of the arrow is proportional to the vector’s magnitude. The arrow’s length is indicated by hash marks in the <a href="#attachment_97591">first figure</a> and <a href="#attachment_97593">second figure</a> above. The arrow points in the same direction as the vector.</p>
<p>For two-dimensional motion, the path of an object can be represented with three vectors: one vector shows the straight-line path between the initial and final points of the motion, one vector shows the horizontal component of the motion, and one vector shows the vertical component of the motion.</p>
<p>The horizontal and vertical components of the motion add together to give the straight-line path. For example, observe the three vectors in the <a href="#attachment_97593">figure above</a>. The first represents a 9-block displacement east. The second represents a 5-block displacement north. These vectors are added to give the third vector, with a 10.3-block total displacement. The third vector is the straight-line path between the two points.</p>
<p>Note that in this example, the vectors that we are adding are perpendicular to each other and thus form a right triangle. This means that we can use the Pythagorean theorem to calculate the magnitude of the total displacement. (Note that we cannot use the Pythagorean theorem to add vectors that are not perpendicular. We will develop techniques for adding vectors having any direction, not just those perpendicular to one another, in the next couple of lessons.</p>
</section>

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Two-Dimensional Motion

Physics is a science discipline concerned with the nature and properties of matter and energy. Physics topics include mechanics, heat, light and radiation, sound, electricity, magnetism, the structure of atoms, and so on.

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Explore the principles of two-dimensional kinematics, including mastering vector addition and subtraction, resolving a vector into components, and analyzing projectile motion and relative velocities.


Two-Dimensional Motion

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