======Strobe Light Circular Saw======


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**Alternative titles:** Freezing a Spinning Saw Blade
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====Summary====                   
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====Procedure====  
  - Place a circular saw (plugged in but not cutting) in a safe demonstration area.  
  - Darken the room enough so the strobe light is effective.  
  - Turn on the saw so the blade rotates freely.  
  - Shine the strobe light directly at the spinning blade.  
  - Adjust the strobe frequency:  
    - At certain frequencies, the blade will appear frozen in place.  
    - If the strobe rate is slightly faster or slower than the blade rotation, the blade appears to move slowly forward or backward.  
  - Discuss how this is an optical illusion created by timing between the flashes and the spinning blade.



====Links==== 

🎞️ STROBE LIGHT SCIENCE - Minuteman High School: [[https://www.facebook.com/watch/?v=5652889581458209]]\\

🎞️ Strobe Light Wood Turning! - pswii360i (similar demonstration with lathe): [[https://www.reddit.com/r/gifs/comments/4wupfy/strobe_light_wood_turning/]]\\

====Variations====
  * Mark one tooth of the blade with chalk or paint to make the effect easier to see.  
  * Try the same effect with a fan or bicycle wheel for a safer classroom demonstration.  
  * Record the effect with a camera to show “aliasing” of motion.



====Safety Precautions====
  * Extreme caution: a circular saw is dangerous even when not cutting.  
  * Only a trained adult should operate the saw.  
  * Keep the saw in a stable, secure position—never bring hands near the spinning blade.  
  * Use protective eyewear.  
  * Consider using a safer alternative (e.g., a box fan, drill-powered disc, or toy propeller) for classroom demonstrations.




====Questions to Consider====
  * Why does the saw blade appear still under the strobe light? (Because the strobe flashes in sync with the rotation, showing the blade in the same position each time.)  
  * Why can the blade appear to spin backward? (If the strobe rate is slightly slower than the blade rotation, each flash shows the blade a bit ahead, creating the illusion of backward motion.)  
  * How is this effect related to movies and video of wheels appearing to spin backwards? (Both are stroboscopic effects caused by sampling motion at discrete intervals.)  
  * Why is using a safer substitute (like a fan) better for classroom demonstrations? (It eliminates the risk of severe injury while showing the same optical principle.)