Unlike an airplane or helicopter propeller, which starts spinning while the vehicle is completely still, you
throw the boomerang, so that in addition to its spinning propeller motion, it also has the motion of flying through the air.
In the diagram below, you can see that whichever wing is at the top
of the spin at any one time ends up moving in the same direction as the
forward motion of the throw, while whichever wing is at the bottom of
the spin is moving in the opposite direction of the throw. This means
that while the wing at the top is
spinning at the same speed as the wing at the bottom, it is actually moving through the air at a higher rate of speed.
When a wing moves through the air more quickly, more air passes under
it. This translates into more lift because the wing has to exert more
force to push down the increased mass. So, it's as if somebody were
constantly pushing the whole spinning propeller of the boomerang at the
top of the spin.
But everybody knows that when you push something
from the top, say a chair, you tip the thing over and it falls to the
ground. Why doesn't this happen when you push on the top of a spinning
boomerang?
If you've read
How Gyroscopes Work,
then you may have already guessed what's going on here. When you push
on one point of a spinning object, such as a wheel, airplane propeller
or boomerang, the object doesn't react in the way you might expect. When
you push a spinning wheel, for example, the wheel reacts to the force
as if you pushed it at a point
90 degrees off from
where you actually pushed it. To see this, roll a bicycle wheel along
next to you and push on it at the top. The wheel will turn to the left
or right, as if there were a force acting on the front of the wheel.
This is because with a spinning object, the point you push isn't
stationary, it's rotating around an axis! You applied the force to a
point at the top of the wheel, but that point immediately moved around
to the front of the wheel while it was still feeling the force you
applied. There's a sort of delayed reaction, and the force actually has
the strongest effect on the object about 90 degrees off from where it
was first applied.
In this scenario, the wheel would quickly straighten out after turning slightly because as the
point of force
rotates around the wheel, it ends up applying force on opposite ends of
the wheel, which balances out the effect of the force. But constantly
pushing on the top of the wheel would keep a steady force acting on the
front of the wheel. This force would be stronger than the
counterbalancing forces, so the wheel would keep turning, traveling in a circle.
If you've ever steered a
bicycle
without using the handlebars, you've experienced this effect. You shift
your weight on the bicycle so that the top of the wheel moves to the
side, but every bicycle rider knows that the bike doesn't tip over as it
would if it were standing still, but turns to the right or left
instead.
This is the same thing that is happening in a boomerang.
The uneven force caused by the difference in speed between the two wings
applies a
constant force at the top of the spinning
boomerang, which is actually felt at the leading side of the spin. So,
like a leaning bicycle wheel, the boomerang is constantly turning to the
left or right, so that it travels in a circle and comes back to its
starting point.
