‘Practical Test’: YouTuber charters chopper to solve physics conundrum

Washington: What shape does a rope dangling from a flying helicopter take? This physics question from a 2014 US exam saw disputes even after the correct solution furnished by the examiners, and science YouTuber Derek Muller set to furnish a conclusive answer – by renting a helicopter to drop a rope himself.

In a video on his Veritasium channel, which has garnered over two million views so far, Muller read out the scenario outlined in the qualifying exam for the US Physics Olympiad team, before setting out to solve it.

The question read: “A helicopter is flying horizontally at a constant speed. A perfectly flexible uniform cable is suspended beneath the helicopter. Air friction on the cable is not negligible. Which of the following diagrams best shows the shape of the cable as the helicopter flies through the air to the right?”

It gave five options – hanging straight down, straight diagonal to the left, hook shaped, inverted hook shaped or S-bended.

Before running his experiment, Muller ran a poll on his channel for viewers to post what they thought was the correct answer, and received various responses – with most plumping for hook shaped and then, straight diagonal to the left. No one chose the S-bend option.

Showing the answer by his experiment, Muller noted that there are two external forces acting on the rope: gravity, which pulls it downwards, and air resistance, moving it to the left. “When flying along at constant speeds, these forces must be perfectly balanced by the tension in the rope,” Muller said.

The exam paper itself, posted on the American Association of Physics Teachers’ (AAPT) website, had noted that “since there is air friction on the cable, then there must be a horizontal component to the force where the cable attaches to the helicopter”.

It stated that “this question generated a great deal of controversy” and that “at least two test-takers challenged the answer”.

Just to make sure, Muller, who used a rope used in gyms initially, repeated the experiment with one carrying a weight, and then one with a parachute attached to it.

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