# Day 4: Velocity of a Wave

Monday, 18 August 2014

The idea I wanted to get across: a wave’s speed depends on the medium through which it travels, not the wave’s frequency or wavelength as $v=\lambda f$ might imply.

The setup: meter sticks, stopwatch, ring stand, string, and a few inches of duct tape.

In class: Get the kids to figure out the speed of a longitudinal pulse with the materials you’ve laid out. Then ask if anyone thinks they can make the wave go faster. Inevitably a kid will say “I got this” and confidently move in to make a higher speed wave. He’ll (and isn’t it always a he?) try with more force or higher frequency and you will see a similar speed. Dude, I love this part. How can we change the medium? Oh, let’s stretch the Slinky more and BINGO! Faster wave. Bonus side-effect: you can use the same setup to show how increasing the frequency will decrease the wavelength of waves on the Slinky.

PS — I absolutely loved writing this 180 blog last year because it was fun to share something cool each day. This year, I struggled to get it going again. The biggest hurdle it turned out (thanks Jonathan) was remembering to take a photo or video daily. The other piece was a fear that everything I do this year will be a repeat of last year. I got over that by remembering it won’t be the same because I tweak stuff every year. Also, it could be fun comparing where I was this day last year. I appreciate all on Twitter who helped me think through this: @sterlace, @fnosches, @gfrblxt, @rawrdimus, @absvalteaching, @justinaion, and @_cuddlefish_.