"Tricky questions" that help to understand physics

Here are two "tricky questions" that are really interesting to help understand physics.

  1. Attach a tennis ball to string, and swing it over your head. The ball follows a circular path. Release the string. What path does the ball follow?
  2. The speed of light is 300,000 km per second, typically shown as the letter c. Take two pieces of wire each 300,000 km long. Place a light bulb and a battery 1 meter apart. Connect the light bulb to each wire. Before connecting the battery, stretch out the wire as far as you can on each side. Now connect the battery. How long until the bulb lights up? 0.5 seconds, 1 sec, 2 secs, or 1/c secs?

Why are these "trick questions"? Because of the assumptions we usually make when thinking about questions like these. Picking apart these assumptions reveals a lot about how physics works.

For the tennis ball on the string, this "All Things Physics" video recently posed the question. I could feel that there was something tricky, but I had to watch the video to realize that the tricky part is the phrase "release the string". Does "release" mean you let go of the string (release from the center)? Or does it mean the ball detaches from the string (release from perimeter)? In the real physical world, the ball takes different paths depending on which type of release! The video shows what happens when you let go of the string, and I highly recommend watching.

For the light bulb question, the tricky part of the question is "bulb lights up". Does "light up" mean the moment that current starts to flow through the bulb? Or does "light up" mean enough current is flowing to cause the bulb to emit light visible to the human eye? The time until each of these is significantly different. YouTube science channel "Veritasium" posed this question in 2021. Here's a video playlist, that begins with the original video. The playlist includes several follow-up videos that actually do the experiment, of course in a slightly more limited form.

These tricky questions demonstrate how we create models that approximate how the real world works. These models work well, until they start to fall apart as circumstances become more extreme. As George Box said, "All models are wrong, but some are useful." [wp]

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