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<p>By the end of this lesson and the next few, you should be able to:</p>
<ul>
<li>Perform unit conversions both in the SI and English units.</li>
<li>Explain the most common prefixes in the SI units and be able to write them in scientific notation.</li>
</ul>
<h2>What is a physical quantity?</h2>
<p>The range of objects and phenomena studied in physics is immense. From the incredibly short lifetime of a nucleus to the age of the Earth, from the tiny sizes of sub-nuclear particles to the vast distance to the edges of the known universe, from the force exerted by a jumping flea to the force between Earth and the Sun, there are enough factors of 10 to challenge the imagination of even the most experienced scientist.</p>
<div class="wp-caption alignnone" id="attachment_70908" style="width: 400px"><a href="https://data.ulearngo.com/assets/e4161a03-16b0-418e-8711-72c0e3d0b4c0"><img alt="earth-to-moon" class="size-full wp-image-70908" height="225" src="https://data.ulearngo.com/assets/e4161a03-16b0-418e-8711-72c0e3d0b4c0" width="400"/></a><p class="wp-caption-text">The distance from Earth to the Moon may seem immense. But it is just a tiny fraction of the distances from Earth to other celestial bodies. Image Attribution: NASA</p></div>
<p>Giving numerical values for physical quantities and equations for physical principles allows us to understand nature much more deeply than does qualitative description alone. To comprehend these vast ranges, we must also have accepted units in which to express them. And we shall find that (even in the potentially mundane discussion of meters, kilograms, and seconds) a profound simplicity of nature appears—all physical quantities can be expressed as combinations of only four fundamental physical quantities: length, mass, time, and electric current.</p>
<p id="import-auto-id1675918">We define a <strong><span data-type="term" id="import-auto-id1536256">physical quantity</span></strong> either by <em data-effect="italics">specifying how it is measured</em> or by <em data-effect="italics">stating how it is calculated</em> from other measurements. For example, we define distance and time by specifying methods for measuring them. On the other hand, we define <em data-effect="italics">average speed</em> by stating that it is calculated as distance traveled divided by time of travel.</p>
<h2>What is a unit?</h2>
<p id="import-auto-id2985261">Measurements of physical quantities are expressed in terms of <strong><span data-type="term" id="import-auto-id1666000">units</span></strong>, which are standardized values. For example, the length of a race is a physical quantity. We can express it in units of meters (for sprinters) or kilometers (for distance runners). Without standardized units, it would be extremely difficult for scientists to express and compare measured values in a meaningful way. (<a href="#attachment_70909">See image below</a>.)</p>
<div class="wp-caption alignnone" id="attachment_70909" style="width: 350px"><a href="https://data.ulearngo.com/assets/73c36b33-247e-47a7-8d3c-b41c13ed691f"><img alt="meaningless-distance" class="size-full wp-image-70909" height="303" src="https://data.ulearngo.com/assets/73c36b33-247e-47a7-8d3c-b41c13ed691f" width="350"/></a><p class="wp-caption-text">Distances given in unknown units are maddeningly useless. Image Attribution: OpenStax College Physics</p></div>
<h2>Systems of Units</h2>
<p>There are two major systems of units used in the world: <strong>SI units</strong> (also known as the metric system) and <span data-type="term">English units</span> (also known as the customary or imperial system). <strong>English units</strong> were historically used in nations once ruled by the British Empire and are still widely used in the United States. Virtually every other country in the world now uses SI units as the standard. The metric system is also the standard system agreed upon by scientists and mathematicians. The acronym “SI” is derived from the French <em data-effect="italics">Système International</em>.</p>

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Physical Quantities and Units

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.

Physics

Explore the fundamental concepts of physics, including the scientific method, physical quantities and units, and accuracy and precision through lessons on applications, models, theories, laws, and significant figures in physics.


Physical Quantities and Units

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