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<section id="attach-arrow">
<h1>attach_arrow()<a class="headerlink" href="#attach-arrow" title="Permalink to this heading">¶</a></h1>
<p>Arrows are often used to visualize vectors such as the velocity of a moving object, or the force acting on the object. <em>attach_arrow</em> allows automatic updating of the position, direction, and size of the arrow as the object moves.</p>
<figure class="align-default" id="id1">
<img alt="_images/attach_arrow.png" src="_images/attach_arrow.png" />
<figcaption>
<p><span class="caption-text">An arrow representing velocity is attached to a bouncing ball.</span><a class="headerlink" href="#id1" title="Permalink to this image">¶</a></p>
</figcaption>
</figure>
<p>Caveats:</p>
<ul class="simple">
<li><p>The quantity to be visualized must be a vector attribute of the moving object.</p></li>
<li><p>The value of the quantity must be updated by the code as the object moves.</p></li>
<li><p>To keep the cross section of the arrow constant, set <em>shaftwidth</em>.</p></li>
</ul>
<dl class="py function">
<dt class="sig sig-object py" id="attach_arrow">
<span class="sig-name descname"><span class="pre">attach_arrow</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">ball</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">"velocity"</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">color=color.green</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">scale=10</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">shaftwidth=ball.radius/3</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#attach_arrow" title="Permalink to this definition">¶</a></dt>
<dd><dl class="field-list simple">
<dt class="field-odd">Parameters<span class="colon">:</span></dt>
<dd class="field-odd"><ul class="simple">
<li><p><strong>firstargument</strong> (<em>object</em>) – Object to which to attach the arrow (<em>ball</em> in example above).</p></li>
<li><p><strong>secondargument</strong> (<em>string</em>) – Name of vector attribute to be represented by the arrow (“velocity” in example above).</p></li>
<li><p><strong>scale</strong> (<em>scalar</em>) – Quantity by which to multiply the attribute in order to adjust the magnitude of the axis of the arrow.</p></li>
<li><p><strong>color</strong> (<em>vector</em>) – Default is the color of the object to which the arrow is attached.</p></li>
<li><p><strong>shaftwidth</strong> – Cross section of the tail of the arrow.</p></li>
</ul>
</dd>
</dl>
</dd></dl>
<section id="turning-attach-arrow-off-and-on">
<h2>Turning <em>attach_arrow</em> off and on<a class="headerlink" href="#turning-attach-arrow-off-and-on" title="Permalink to this heading">¶</a></h2>
<p>To start and stop displaying the arrow:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">ball</span> <span class="o">=</span> <span class="n">sphere</span><span class="p">(</span><span class="n">color</span><span class="o">=</span><span class="n">color</span><span class="o">.</span><span class="n">yellow</span><span class="p">,</span> <span class="n">velocity</span><span class="o">=</span><span class="n">vec</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">)</span> <span class="p">)</span>
<span class="n">myarrow</span> <span class="o">=</span> <span class="n">attach_arrow</span><span class="p">(</span> <span class="n">ball</span><span class="p">,</span> <span class="s2">"velocity"</span><span class="p">,</span> <span class="n">scale</span><span class="o">=</span><span class="mf">2.0</span><span class="p">)</span>
<span class="o">...</span>
<span class="n">myarrow</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span> <span class="c1"># stops arrow display</span>
<span class="o">...</span>
<span class="n">myarrow</span><span class="o">.</span><span class="n">start</span><span class="p">()</span> <span class="c1"># restarts arrow display</span>
</pre></div>
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