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    <p>If you are trying to emulate friction, the easiest way, is to <strong>know the velocity</strong> and use a mathematical model, or <em>differential equation</em> to approximate it.</p> <pre><code> dx -c * -- dt </code></pre> <p>basically, the force of friction is proportional to the velocity and in opposite direction. Thus, you can calculate the force of friction (or drag) simply as follows:</p> <pre><code>objects[i]-&gt;force = -drag_coefficient * objects[i]-&gt;velocity </code></pre> <p>If you do not know the velocity, it gets difficult to get good results, as you may have a force pushing right, while the object is moving left (therefore you would apply drag to the wrong direction).</p> <h3>Edit:</h3> <p>Actually, not all kinds of friction depend on the <em>magnitude (speed) of the velocity</em>, <strong>dry friction</strong> is often referred as just taking the <em>direction of the velocity</em> into account:</p> <pre><code>force = -drag_coefficient * sign(velocity) </code></pre> <p>However computationally this becomes unstable as velocity reaches 0, overshoots it and oscillates back and forth.</p> <p>Another model is the one for <strong>wet friction</strong>, that is the one I used in the first example:</p> <pre><code>force = -drag_coefficient * velocity </code></pre> <p>This means that as your object is running faster, it will be slowed down more by friction. Think for example of the force of air, as you are running, the faster you are running, the more the air is trying to slow you down.</p> <p>Neither of the models above is 100% accurate, but for a game they should be more than enough</p>
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    1. COYou are right, I am not a physicist, though I like solving differential equations numerically. As for a game, I think that these problem can mostly be solved with any model that behaves roughly right , and can be seen as kind of an artistic freedom. If true accuracy is required, I wolud utilize an off-the-shelf physics engine, that already has most of this stuff implemented.
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    2. COWould that I could, but my assignment is to MAKE a physics engine :/
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    3. COThank you Decden :D I'm just so surprised that I didn't need to use the drag equation (1/2)*airDensity*speed^2*DC*area! It works perfectly :D
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