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a while<font color="#CCCCCC"> back I was</font><font color="#E5E5E5"> teaching physics in</font>

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California<font color="#CCCCCC"> I got to class</font><font color="#E5E5E5"> and I was all</font>

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like<font color="#CCCCCC"> hey class you</font><font color="#E5E5E5"> want to hear a</font>

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physics joke<font color="#E5E5E5"> yeah okay</font><font color="#CCCCCC"> totally</font><font color="#E5E5E5"> does</font>

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light hold mass I don't know it does<font color="#E5E5E5"> it</font>

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<font color="#E5E5E5">no it's</font><font color="#CCCCCC"> not</font><font color="#E5E5E5"> even Catholic</font><font color="#CCCCCC"> oh</font><font color="#E5E5E5"> man she</font>

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went to the beach it is<font color="#E5E5E5"> really</font><font color="#CCCCCC"> totally</font>

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lame<font color="#CCCCCC"> but wait</font><font color="#E5E5E5"> doesn't light really have</font>

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mass it should

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it's called momentum so it's gotta have

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mass it adds momentum<font color="#CCCCCC"> so it</font><font color="#E5E5E5"> must have</font>

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mass whoa<font color="#E5E5E5"> hey everyone just calm</font><font color="#CCCCCC"> the</font>

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heck down<font color="#CCCCCC"> you see</font><font color="#E5E5E5"> the worst part of all</font>

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<font color="#E5E5E5">this wasn't that my joke bombed</font><font color="#CCCCCC"> it was</font>

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that I actually<font color="#E5E5E5"> managed to confuse</font>

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<font color="#CCCCCC">people by telling it my students had</font>

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<font color="#CCCCCC">read that light has momentum</font><font color="#E5E5E5"> and they</font>

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were right<font color="#CCCCCC"> light really</font><font color="#E5E5E5"> does have</font>

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momentum<font color="#E5E5E5"> but then I come</font><font color="#CCCCCC"> to class and</font>

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I'm all like<font color="#CCCCCC"> hey</font><font color="#E5E5E5"> guess what light has no</font>

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<font color="#E5E5E5">mass now my students are</font><font color="#CCCCCC"> thinking to</font>

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<font color="#E5E5E5">themselves dude but P equals</font><font color="#CCCCCC"> MV in other</font>

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words if momentum equals mass<font color="#E5E5E5"> times</font>

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<font color="#E5E5E5">velocity</font><font color="#CCCCCC"> how could light which has no</font>

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mass ever have momentum<font color="#E5E5E5"> well I had to</font>

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break<font color="#E5E5E5"> it to my students</font><font color="#CCCCCC"> that</font><font color="#E5E5E5"> P</font><font color="#CCCCCC"> equals MV</font>

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isn't really accurate for things that

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travel close to the speed of<font color="#E5E5E5"> life for</font>

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<font color="#E5E5E5">things going that fast you have</font><font color="#CCCCCC"> to use</font>

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<font color="#E5E5E5">special relativity</font><font color="#CCCCCC"> I don't want to waste</font>

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a lot<font color="#CCCCCC"> of time talking about</font><font color="#E5E5E5"> special</font>

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<font color="#CCCCCC">relativity in this video so you're just</font>

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<font color="#E5E5E5">gonna have to take my word for it that</font>

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<font color="#E5E5E5">the rules of special relativity allow</font>

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for a loophole<font color="#E5E5E5"> this loophole provides a</font>

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way for massless<font color="#E5E5E5"> objects to have</font>

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momentum<font color="#CCCCCC"> all</font><font color="#E5E5E5"> right so the bad news is</font>

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that<font color="#E5E5E5"> we cannot use</font><font color="#CCCCCC"> P equals mV</font>

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to find the<font color="#CCCCCC"> momentum of a photon</font><font color="#E5E5E5"> the</font>

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<font color="#CCCCCC">good news is that the formula for the</font>

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<font color="#E5E5E5">momentum of a photon is simple the</font>

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<font color="#E5E5E5">momentum of a photon equals H over</font>

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lambda H is Planck's constant<font color="#CCCCCC"> 6.6 to 6</font>

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times 10 to<font color="#CCCCCC"> the negative 34</font>

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joule-seconds lambda is the wavelength

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of the<font color="#CCCCCC"> light in meters be careful don't</font>

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<font color="#E5E5E5">use nanometers you have to convert to</font>

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<font color="#CCCCCC">meters</font>

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the<font color="#E5E5E5"> momentum of a single photon</font><font color="#CCCCCC"> is</font><font color="#E5E5E5"> going</font>

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<font color="#E5E5E5">to be extremely small</font><font color="#CCCCCC"> that's why it</font>

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doesn't<font color="#CCCCCC"> feel like we're getting</font><font color="#E5E5E5"> pushed</font>

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on very much when light shines<font color="#CCCCCC"> on us</font>

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but<font color="#E5E5E5"> theoretically</font><font color="#CCCCCC"> if you had a big</font>

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enough solar sail the light bouncing<font color="#E5E5E5"> off</font>

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<font color="#CCCCCC">of that solar sail</font><font color="#E5E5E5"> could propel it</font>

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forward<font color="#E5E5E5"> due to</font><font color="#CCCCCC"> the impulse imparted by</font>

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<font color="#E5E5E5">the momentum of the light</font><font color="#CCCCCC"> okay so</font><font color="#E5E5E5"> very</font>

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long story short you<font color="#CCCCCC"> should</font><font color="#E5E5E5"> never ever</font>

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<font color="#CCCCCC">use</font><font color="#E5E5E5"> P equals</font><font color="#CCCCCC"> MV</font><font color="#E5E5E5"> to find the momentum of</font>

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a photon to find the<font color="#CCCCCC"> momentum of a</font>

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photon you<font color="#E5E5E5"> should always use</font><font color="#CCCCCC"> momentum</font>

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equals H<font color="#CCCCCC"> over lambda</font>