Posted on 02 January 2012 at 12:47h

The other day I was sent a link to this interesting website - Veritasium. There is some great stuff here. My students have done a lot of experiments with slinkies so I was particularly interested in the slinky drop experiments. I like the explanation about the wave travelling at the same speed as the end of the slinky but couldn't quite tie it in with Hooks law. If the ball is at rest then the tension = the weight but as the spring gets shorter the tension must get less. I tried simulating this in interactive physics by placing two springs between 3 balls but it didn't work. I also copied the video into logger pro and found that the top accelerates at a little faster then g. The middle seems to accelerate at much less then g but I only had a few frames to analyse so won't dwell on that. After a lot of playing around it suddenly all made sense. I was making the mistake of looking at the tension in the whole spring but I should have just considered the bottom coil. The separation of the last two coils is constant until the top comes down to meet it, the tension is related to the coil separation so will also be constant.

After further experimentation this time with Algodoo I made a working simulation.

Notice how the last section doesn't change length until the compression wave gets to the bottom. I also sent a wave down the slinky without cutting it. The time for the wave to get down the slinky is about the same as the time for the top of the slinky to drop.