{"id":633,"date":"2014-12-11T02:30:07","date_gmt":"2014-12-11T02:30:07","guid":{"rendered":"https:\/\/courses.candelalearning.com\/colphysics\/?post_type=chapter&#038;p=633"},"modified":"2016-01-21T15:31:28","modified_gmt":"2016-01-21T15:31:28","slug":"4-2-newtons-first-law-of-motion-inertia","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-physics\/chapter\/4-2-newtons-first-law-of-motion-inertia\/","title":{"raw":"Newton\u2019s First Law of Motion: Inertia","rendered":"Newton\u2019s First Law of Motion: Inertia"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n\t<li>Define mass and inertia.<\/li>\r\n\t<li>Understand Newton's first law of motion.<\/li>\r\n<\/ul>\r\n<\/div>\r\nExperience suggests that an object at rest will remain at rest if left alone, and that an object in motion tends to slow down and stop unless some effort is made to keep it moving. What <em> Newton\u2019s first law of motion<\/em> states, however, is the following:\r\n<div>\r\n<div>\r\n<div class=\"textbox shaded\">\r\n<h3>Newton\u2019s First Law of Motion<\/h3>\r\n<section>\r\n<div data-type=\"note\">A body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.<\/div>\r\n<\/section><\/div>\r\n<\/div>\r\nNote the repeated use of the verb \u201cremains.\u201d We can think of this law as preserving the status quo of motion.\r\n\r\nRather than contradicting our experience, <em> Newton\u2019s first law of motion<\/em> states that there must be a <em>cause<\/em> (which is a net external force) <em>for there to be any change in velocity (either a change in magnitude or direction)<\/em>. We will define <em>net external force<\/em> in the next section. An object sliding across a table or floor slows down due to the net force of friction acting on the object. If friction disappeared, would the object still slow down?\r\n\r\nThe idea of cause and effect is crucial in accurately describing what happens in various situations. For example, consider what happens to an object sliding along a rough horizontal surface. The object quickly grinds to a halt. If we spray the surface with talcum powder to make the surface smoother, the object slides farther. If we make the surface even smoother by rubbing lubricating oil on it, the object slides farther yet. Extrapolating to a frictionless surface, we can imagine the object sliding in a straight line indefinitely. Friction is thus the <em>cause<\/em> of the slowing (consistent with Newton\u2019s first law). The object would not slow down at all if friction were completely eliminated. Consider an air hockey table. When the air is turned off, the puck slides only a short distance before friction slows it to a stop. However, when the air is turned on, it creates a nearly frictionless surface, and the puck glides long distances without slowing down. Additionally, if we know enough about the friction, we can accurately predict how quickly the object will slow down. Friction is an external force.\r\n\r\nNewton\u2019s first law is completely general and can be applied to anything from an object sliding on a table to a satellite in orbit to blood pumped from the heart. Experiments have thoroughly verified that any change in velocity (speed or direction) must be caused by an external force. The idea of <em>generally applicable or universal laws<\/em> is important not only here\u2014it is a basic feature of all laws of physics. Identifying these laws is like recognizing patterns in nature from which further patterns can be discovered. The genius of Galileo, who first developed the idea for the first law, and Newton, who clarified it, was to ask the fundamental question, \u201cWhat is the cause?\u201d Thinking in terms of cause and effect is a worldview fundamentally different from the typical ancient Greek approach when questions such as \u201cWhy does a tiger have stripes?\u201d would have been answered in Aristotelian fashion, \u201cThat is the nature of the beast.\u201d True perhaps, but not a useful insight.\r\n<h2>Mass<\/h2>\r\nThe property of a body to remain at rest or to remain in motion with constant velocity is called <em> inertia<\/em>. Newton\u2019s first law is often called the <em> law of inertia<\/em>. As we know from experience, some objects have more inertia than others. It is obviously more difficult to change the motion of a large boulder than that of a basketball, for example. The inertia of an object is measured by its <em> mass<\/em>. Roughly speaking, mass is a measure of the amount of \u201cstuff\u201d (or matter) in something. The quantity or amount of matter in an object is determined by the numbers of atoms and molecules of various types it contains. Unlike weight, mass does not vary with location. The mass of an object is the same on Earth, in orbit, or on the surface of the Moon. In practice, it is very difficult to count and identify all of the atoms and molecules in an object, so masses are not often determined in this manner. Operationally, the masses of objects are determined by comparison with the standard kilogram.\r\n\r\n<\/div>\r\n<div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Check Your Understanding<\/h3>\r\nWhich has more mass: a kilogram of cotton balls or a kilogram of gold?\r\n<div>\r\n\r\n<strong>Solution<\/strong>\r\n\r\n<\/div>\r\nThey are equal. A kilogram of one substance is equal in mass to a kilogram of another substance. The quantities that might differ between them are volume and density.\r\n\r\n<\/div>\r\n<\/div>\r\n<section data-depth=\"1\">\r\n<h2 data-type=\"title\">Section Summary<\/h2>\r\n<ul>\r\n\t<li>Newton\u2019s first law of motion states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia.<\/li>\r\n\t<li>Inertia is the tendency of an object to remain at rest or remain in motion. Inertia is related to an object\u2019s mass.<\/li>\r\n\t<li>Mass is the quantity of matter in a substance.<\/li>\r\n<\/ul>\r\n<\/section><section data-depth=\"1\" data-element-type=\"conceptual-questions\">\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Conceptual Questions<\/h3>\r\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\r\n<div data-type=\"problem\">\r\n\r\n1. How are inertia and mass related?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\r\n<div data-type=\"problem\">\r\n\r\n2. What is the relationship between weight and mass? Which is an intrinsic, unchanging property of a body?\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/section>\r\n<div data-type=\"glossary\">\r\n<h2 data-type=\"glossary-title\">Glossary<\/h2>\r\n<dl><dt>inertia:<\/dt><dd>the tendency of an object to remain at rest or remain in motion<\/dd><\/dl><dl><dt>law of inertia:<\/dt><dd>see Newton\u2019s first law of motion<\/dd><\/dl><dl><dt>mass:<\/dt><dd>the quantity of matter in a substance; measured in kilograms<\/dd><\/dl><dl><dt>Newton\u2019s first law of motion:<\/dt><dd>a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force; also known as the law of inertia<\/dd><\/dl><\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Define mass and inertia.<\/li>\n<li>Understand Newton&#8217;s first law of motion.<\/li>\n<\/ul>\n<\/div>\n<p>Experience suggests that an object at rest will remain at rest if left alone, and that an object in motion tends to slow down and stop unless some effort is made to keep it moving. What <em> Newton\u2019s first law of motion<\/em> states, however, is the following:<\/p>\n<div>\n<div>\n<div class=\"textbox shaded\">\n<h3>Newton\u2019s First Law of Motion<\/h3>\n<section>\n<div data-type=\"note\">A body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.<\/div>\n<\/section>\n<\/div>\n<\/div>\n<p>Note the repeated use of the verb \u201cremains.\u201d We can think of this law as preserving the status quo of motion.<\/p>\n<p>Rather than contradicting our experience, <em> Newton\u2019s first law of motion<\/em> states that there must be a <em>cause<\/em> (which is a net external force) <em>for there to be any change in velocity (either a change in magnitude or direction)<\/em>. We will define <em>net external force<\/em> in the next section. An object sliding across a table or floor slows down due to the net force of friction acting on the object. If friction disappeared, would the object still slow down?<\/p>\n<p>The idea of cause and effect is crucial in accurately describing what happens in various situations. For example, consider what happens to an object sliding along a rough horizontal surface. The object quickly grinds to a halt. If we spray the surface with talcum powder to make the surface smoother, the object slides farther. If we make the surface even smoother by rubbing lubricating oil on it, the object slides farther yet. Extrapolating to a frictionless surface, we can imagine the object sliding in a straight line indefinitely. Friction is thus the <em>cause<\/em> of the slowing (consistent with Newton\u2019s first law). The object would not slow down at all if friction were completely eliminated. Consider an air hockey table. When the air is turned off, the puck slides only a short distance before friction slows it to a stop. However, when the air is turned on, it creates a nearly frictionless surface, and the puck glides long distances without slowing down. Additionally, if we know enough about the friction, we can accurately predict how quickly the object will slow down. Friction is an external force.<\/p>\n<p>Newton\u2019s first law is completely general and can be applied to anything from an object sliding on a table to a satellite in orbit to blood pumped from the heart. Experiments have thoroughly verified that any change in velocity (speed or direction) must be caused by an external force. The idea of <em>generally applicable or universal laws<\/em> is important not only here\u2014it is a basic feature of all laws of physics. Identifying these laws is like recognizing patterns in nature from which further patterns can be discovered. The genius of Galileo, who first developed the idea for the first law, and Newton, who clarified it, was to ask the fundamental question, \u201cWhat is the cause?\u201d Thinking in terms of cause and effect is a worldview fundamentally different from the typical ancient Greek approach when questions such as \u201cWhy does a tiger have stripes?\u201d would have been answered in Aristotelian fashion, \u201cThat is the nature of the beast.\u201d True perhaps, but not a useful insight.<\/p>\n<h2>Mass<\/h2>\n<p>The property of a body to remain at rest or to remain in motion with constant velocity is called <em> inertia<\/em>. Newton\u2019s first law is often called the <em> law of inertia<\/em>. As we know from experience, some objects have more inertia than others. It is obviously more difficult to change the motion of a large boulder than that of a basketball, for example. The inertia of an object is measured by its <em> mass<\/em>. Roughly speaking, mass is a measure of the amount of \u201cstuff\u201d (or matter) in something. The quantity or amount of matter in an object is determined by the numbers of atoms and molecules of various types it contains. Unlike weight, mass does not vary with location. The mass of an object is the same on Earth, in orbit, or on the surface of the Moon. In practice, it is very difficult to count and identify all of the atoms and molecules in an object, so masses are not often determined in this manner. Operationally, the masses of objects are determined by comparison with the standard kilogram.<\/p>\n<\/div>\n<div>\n<div class=\"textbox key-takeaways\">\n<h3>Check Your Understanding<\/h3>\n<p>Which has more mass: a kilogram of cotton balls or a kilogram of gold?<\/p>\n<div>\n<p><strong>Solution<\/strong><\/p>\n<\/div>\n<p>They are equal. A kilogram of one substance is equal in mass to a kilogram of another substance. The quantities that might differ between them are volume and density.<\/p>\n<\/div>\n<\/div>\n<section data-depth=\"1\">\n<h2 data-type=\"title\">Section Summary<\/h2>\n<ul>\n<li>Newton\u2019s first law of motion states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia.<\/li>\n<li>Inertia is the tendency of an object to remain at rest or remain in motion. Inertia is related to an object\u2019s mass.<\/li>\n<li>Mass is the quantity of matter in a substance.<\/li>\n<\/ul>\n<\/section>\n<section data-depth=\"1\" data-element-type=\"conceptual-questions\">\n<div class=\"textbox key-takeaways\">\n<h3>Conceptual Questions<\/h3>\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\n<div data-type=\"problem\">\n<p>1. How are inertia and mass related?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\n<div data-type=\"problem\">\n<p>2. What is the relationship between weight and mass? Which is an intrinsic, unchanging property of a body?<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<div data-type=\"glossary\">\n<h2 data-type=\"glossary-title\">Glossary<\/h2>\n<dl>\n<dt>inertia:<\/dt>\n<dd>the tendency of an object to remain at rest or remain in motion<\/dd>\n<\/dl>\n<dl>\n<dt>law of inertia:<\/dt>\n<dd>see Newton\u2019s first law of motion<\/dd>\n<\/dl>\n<dl>\n<dt>mass:<\/dt>\n<dd>the quantity of matter in a substance; measured in kilograms<\/dd>\n<\/dl>\n<dl>\n<dt>Newton\u2019s first law of motion:<\/dt>\n<dd>a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force; also known as the law of inertia<\/dd>\n<\/dl>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-633\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>College Physics. <strong>Authored by<\/strong>: OpenStax College. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/031da8d3-b525-429c-80cf-6c8ed997733a\/College_Physics\">http:\/\/cnx.org\/contents\/031da8d3-b525-429c-80cf-6c8ed997733a\/College_Physics<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Located at License<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":5,"menu_order":3,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"College Physics\",\"author\":\"OpenStax College\",\"organization\":\"\",\"url\":\"http:\/\/cnx.org\/contents\/031da8d3-b525-429c-80cf-6c8ed997733a\/College_Physics\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Located at License\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-633","chapter","type-chapter","status-publish","hentry"],"part":7458,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/chapters\/633","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/wp\/v2\/users\/5"}],"version-history":[{"count":13,"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/chapters\/633\/revisions"}],"predecessor-version":[{"id":10439,"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/chapters\/633\/revisions\/10439"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/parts\/7458"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/chapters\/633\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/wp\/v2\/media?parent=633"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/pressbooks\/v2\/chapter-type?post=633"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/wp\/v2\/contributor?post=633"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-physics\/wp-json\/wp\/v2\/license?post=633"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}