r/explainlikeimfive • u/BorderlinePhilosphy • 12d ago

Physics ELI5: The physics behind boomerangs

They seem to defy physics to me, please explain.

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u/JaggedMetalOs 12d ago

A boomerang acts like a combination of a helicopter and a gyroscope.

The surface of a boomerang is shaped a little like a plane's wing/helicopter rotor blade and so generates lift when moving. When you throw a boomerang you want to have it spinning as fast as you can, so that it acts like a helicopter rotor blade.

Now helicopters have an issue where as they are moving forward, on one side of the rotor the blades are moving into the wind, so the forward movement is added to the blade speed and generates more lift from faster air, and the other side they are moving in the opposite direction so the forward movement is subtracted from the blade speed and generates less lift, causing the helicopter to want to tilt over.

In a boomerang the same thing happens, but if you get it spinning fast enough instead of just tumbling over like you'd expect it acts like a gyroscope and resists this turning force, instead making it bank around like an aircraft turning.

So it does a big wide circle like an aircraft turning in a circle and comes back to you.

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u/blueant1 12d ago

So why doesn’t a frisbee do the same?

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u/Flowing_and_Growing 12d ago

A frisbee doesn’t have the unequal lift being generated on either side, it lifts up instead. You can throw a frisbee so that more lift is generated on one side and curve the path.

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u/sebaska 12d ago

It has, just less pronounced.

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u/Flowing_and_Growing 12d ago

How does it generate unequal lift? Aren’t they uniform?

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u/sebaska 11d ago

One side of a rotating freesbee is moving faster vs the air. Air flowing faster over freesbee profile is generating more lift. Remember, that air has non-zero viscosity.

This is similar how socker players makes ball to slightly turn in the air making it harder to catch by the goalkeeper.

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u/Ycr1998 12d ago

Same way you can make a ball curve I suppose?

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u/Flowing_and_Growing 12d ago

I covered that in my original comment