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165 lines
7.8 KiB
HTML
165 lines
7.8 KiB
HTML
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<article id="content" role="main"><!-- #BeginEditable "Body" -->
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<h1 align="center">Weight or Mass?</h1>
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<p><b>Q</b>: Aren't "weight" and "mass" the same?</p>
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<p><b>A</b>: Not really.</p>
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<table border="0" align="center">
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<tbody>
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<tr>
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<td> <img src="images/weight-normal.svg" alt="weight normally shows 100"></td>
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<td> </td>
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<td>
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<p>An object <b>has mass</b> (say 100 kg). </p>
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<p>This makes it heavy enough to show a <b>weight</b> of "100 kg".</p></td>
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</tr>
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</tbody></table>
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<p>
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But the scales are only showing a guess of the mass above them!
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</p>
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<h2>Gravity causes Weight</h2>
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<p style="float:left; margin: 0 10px 5px 0;"><img src="images/gravity-guy.svg" alt="gravity"> </p>
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<p>An object's <strong>weight</strong> is how hard gravity is pulling on it. </p>
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<p>We think the weight is the same everywhere ... because we all live on the surface of the planet Earth!</p>
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<div style="clear:both"></div>
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/weight-freefall.svg" alt="freefall weight is zero"></p>
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<div align="right">
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<p><b>But</b> in orbit it would not push on the scales at all.</p>
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<p>The scales would show <b>0</b> ... <br>
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... but the mass is still <b>100 kg</b> !</p>
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</div>
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<br>
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<p>An object's <b>mass doesn't change</b> (unless you remove some!), but its <b>weight can change</b>.</p>
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<div style="clear:both"></div>
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/weight-moon.svg" alt="freefall weight is zero"></p>
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<div align="right">
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<p>On the Moon the scales would <i>wrongly </i>show <b>16.6</b><br>for a mass of <b>100 kg</b></p>
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<p>Because the pull of gravity on the Moon<br>is much less than on Earth </p></div><br>
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<div style="clear:both">
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</div>
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<h2>So Why Do People Say Weight instead of Mass?</h2>
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<p><b>People often use "weight" to mean "mass"</b>, and vice versa, because <a href="../physics/gravity.html">Gravity</a> is almost the same everywhere on Earth and we don't notice a difference.</p>
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<p align="center"><i>But remember .. they do not mean the same thing, <br>
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and they <b>can</b> have different measurements.</i></p>
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<h2>Weight is a Force</h2>
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<p>So ... if weight and mass are different, why are they both in kilograms?</p>
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<p class="large" align="center">Well, weight should not really be in kilograms!</p>
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<p>I have used "kilogram" so far because that is what you see on a pair of scales, but it is <b>technically wrong to talk about weight in kilograms</b> ... </p>
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<p class="indent50px">... weight is a <a href="../physics/force.html">force</a> ...</p>
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<p class="indent50px">... which is measured in <b>Newtons</b></p>
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<h2>Newtons</h2>
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<p>The correct <a href="unit.html">unit</a> for force is the <b>Newton</b> (=1 kg·m/s<sup>2</sup>) which is abbreviated <b>N</b>. </p>
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<table border="0" align="center">
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<tbody>
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<tr>
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<td><img src="../images/style/down.gif" alt="down" height="46" width="46"></td>
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<td align="center">On the Earth's surface gravity makes a<br> <b>1 kilogram mass</b> exert about <b>9.8 Newtons of force</b></td>
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</tr>
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</tbody></table>
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<p class="larger">So a 100 kg mass really weighs about 980 Newtons on Earth.</p>
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<p> </p>
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<p style="float:left; margin: 0 10px 5px 0;"><img src="images/scales-newtons.gif" alt="scales in newtons" height="257" width="122"></p>
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<h3>Why Do Scales Show Kilograms?</h3>
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<p>Scales show Kilograms because that is what people understand best ...</p>
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<p align="right">... but it is really just an <b>estimate of the mass</b> above them.</p>
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<p>Scales should really show Newtons, but that might confuse people!</p>
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<p> </p>
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<p><i>Question: how many Newtons should the scales show when <b>you</b> stand on them (hint: multiply kg by 9.8)?</i></p>
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<p> </p>
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<div class="bigul">
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<ul>
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<li>So the scales show an <b>estimate of your mass</b> based on the force your body exerts on it.</li>
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<li>And to find out how much force your body is exerting on the scales, multiply by 9.8 (to convert kg into Newtons).</li>
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</ul>
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</div>
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<h2>Apparent Weight</h2>
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<p>But scales can be fooled ... because they measure a "downwards force" and don't know if it is gravity or some other force! </p>
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<p class="center80"><i>Just jump up and down (gently!) on your scales at home to see your apparent weight change, while your mass stays the same.</i></p>
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<p>So your <b>mass</b> is the same, and your <b>weight</b> is the same (because the force of gravity hasn't changed), but your <b>"apparent" weight changes</b>. Read more at <a href="apparent-weight.html">Apparent Weight</a></p>
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<h2>Conclusion</h2>
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<div class="bigul">
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<ul>
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<li><b>Mass</b> is a measure of how much <b>matter</b> something contains</li>
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<li><b>Weight</b> is a measure of how strongly gravity pulls </li>
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<li><b>Apparent Weight</b> is a measure of downwards force</li>
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<li><b>Force</b> is measured in <b>Newtons</b>, not kilograms</li>
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<li>When scales show "kg" it is just an <b>estimate</b> of the mass above them</li>
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</ul>
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</div>
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<p> </p>
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<script type="text/javascript">getQ(1010, 1011, 1950, 1951, 1952, 1953, 3642, 3643, 3644, 3645);</script> </div>
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<div class="related"> <a href="../unit-conversion-tool.html">Unit Converter</a> <a href="index.html">Measurement Index</a> </div>
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