Slowly Pulling My Way

Turns out I've been teaching for 17 years now.  Wow!  Since most days I'm hanging with (and acting like) 17 year-olds, it's like I've been doing this job all my life!

So, you'd think I'd be better at it, right?

I've been trying to design a good pulley lab for all 17 years.  You know, the old block-n-tackle simple machine.  It's an easy concept but usually a real eye opener ("Wait, each point of contact of the string exerts its own force on the load?").


For years, I had kids measure the various forces with spring scales, then I had them measure them with Vernier force probes.  Then I had them measure the work done in lifting the same load in multiple ways.  Invariably, the students would get caught up in the slight imperfections:  something that should be a force multiple of 5 would come out as 4.8 or they would calculate the pulley system was 95 % efficient or something.  Then I set up stations they could walk up to and read off the forces so they couldn't mess it up.  It was never effective!

All I ever wanted was for them to experience the force multiplier as the number of points-of-contact of the string.

Finally, this year, I just strung up two block and tackle machines side-by-side.  One had three windings and one only had a single string.  I asked them to pull on both at the same time and compare how much force they had to exert while noting how each one went up a different amount.

One is three times easier to pull but only goes up 1/3 as much for the same pull.  No calculations or measurements, just a qualitative question or two.  Instead of frustration, I got "That was a good activity - I think I get it..."

Sometime a simple experience is the best explainer for a simple machine.

Imagine how good I'll be in another 17 years...