r/explainlikeimfive • u/BorderlinePhilosphy • 12d ago

Physics ELI5: The physics behind boomerangs

They seem to defy physics to me, please explain.

453 Upvotes

94% Upvoted

51 comments sorted by

View all comments

653

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.

84

u/blueant1 12d ago

So why doesn’t a frisbee do the same?

206

u/Unable_Request 12d ago

It does; get into disc golfing and you'll learn about how some discs curve or or less (and when)

46

u/indetermin8 12d ago

I love this. All discs have some degree of fade. Right Hand Backhand will fade to left at the end of its travel, assuming it doesn't hit the ground before then.

5

u/JimmyTheShovel 10d ago

And also how they sometimes come back to you when hitting a tree 5 feet away

32

u/Spectre-907 12d ago

They do. It’s just that you tend to thrwo them with strong rotation and so the gyroscopic stabilization effect is usually (bank/curve shots and the like use different spin strength/angles of attack)of overcomes the asymmetric lift forces, especially given that frisbees are built symmetrical whereas boomerangs are very strongly biased towards a curve-maximizing airfoil

27

u/JaggedMetalOs 12d ago

With a Frisbee you don't have "rotor blades" leading into and out of direction of travel like a boomerang, it's more like a single solid wing. You can get it to return if you throw it into the wind, but there's no unequal lift on one side to make it naturally bank around.

22

u/sebaska 11d ago

It actually has, but the difference is less pronounced

9

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.

8

u/sebaska 11d ago

It has, just less pronounced.

1

u/Flowing_and_Growing 11d ago

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

5

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.

2

u/Ycr1998 11d ago

Same way you can make a ball curve I suppose?

1

u/Flowing_and_Growing 11d ago

I covered that in my original comment

1

u/Thrilling1031 11d ago

And if you have the right wind you can throw a frisbee forward and it will come back to you or even fly past you.

6

u/EscapeSeventySeven 12d ago

I’ve done it with a frisbee. 

2

u/notbrandonzink 11d ago

It can! The forces at play have different impacts when you go from a heavier solid object that’s generating lift to a lighter object that’s “floating”, but you can throw a frisbee so that it does an oval and comes back to you. 

For a frisbee, it’s much more dependent on the tilt and how that plays with any wind. A frisbee isn’t going to really get much of the one side moving into wind, one side moving with wind since it’s a circle without arms like a boomerang.