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Javelin "Dip Angle" Change
Name: Justin C.
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
A student of mine who participates in the javelin event in
track and field posed a question that I do not have any idea of how
to answer. We have spoken about gyroscopes and torques. When the
javelin is thrown, it spins several times a second. Why does the
javelin, initially aimed above the horizontal, come down aimed below
the horizontal? Should it not maintain its original orientation
since it is spinning? Now since we know it does change its "dip
angle", and it does not matter what hemisphere or the handedness of
the throw, what force (torque) is acting on it to create this change
in direction.
Parenthetically, I have seen this same pattern with footballs. On a
search for footballs, I have come up with no answers.
Replies:
I'm not a javelin expert, but I did a little research and found out that the
Javelin is weighted unevenly to intentionally cause the downward rotation.
The center of mass is ahead of the center of drag, resulting in a forward
'torque'. The javelin was re-designed a couple decades ago to have its
center of mass slightly forward, causing it to pitch downward during flight.
This was done for safety reasons, both to reduce the throw distance, and
also because a point-down landing means it will not skip or bounce. Before
the change, the javelin did not rotate down, and often would land
horizontally or tip-up.
The case of the football is quite different. For the football, the throwing
mechanics are very different than the javelin. The aerodynamics are quite
different too due to its greater drag and lower density. Depending on how
and how fast the football is rotating, whether the rotation (spiral) is
perfectly around its center, or slightly off center, you get different
aerodynamics for the same ball. The throwing motion of the football can
impart a slight nosing down motion that can explain the nosing down you
occasionally see, but not all throws nose down either. Perhaps there is some
small aerodynamic effect that causes the ball to nose down naturally, but I
would guess the nature of the throwing motion far outweighs that effect.
This is just speculation, so hopefully another respondent will give more
information.
Hope this helps,
Burr
Justin,
I suspect pressure and fluid flow, specifically air, are at hand here.
Although this explanation is two dimensional, it can be imagined in
three dimensions. Consider the javelin or football when thrown. The
initial velocity is along the axis. Air flow above and below the object
are the same. The spinning prohibits random motion. As the velocity
becomes a little more horizontal (projectile motion), air flow is no
longer symmetric. Air going over the top is closer to the object near
the front end, pulling away a little near the back end. Air going under
the bottom does just the reverse. It must compress a little near the
back end to go around. This provides a little net downward pressure in
front and a little upward pressure in back, giving the object's axis a
tendency to line up with the object's velocity. An object that is not
long and narrow, a baseball for example, would not experience this
effect.
Due to the behavior of gyroscopes, one might expect this to cause a form
of precession, a slight turn to the side. Should this begin, the air
pressure effects on the sides of the object would prevent the
precession. Consider a vector analysis of a javelin thrown with the
right hand and moving to the left side of the screen. Angular momentum
points forward, along the axis. Air pressure creates a torque out of
the screen, tending to make the angular momentum rotate out of the
screen. The front of the javelin starts to move out of the screen. The
resulting air pressure then causes a torque downward, perpendicular to
the javelin's axis.
It is possible to allow for both torques. The downward torque causes
the forward angular momentum to shift downward. The torque out of the
page causes the football itself to rotate, keeping it in line with the
angular momentum. I believe this to be the physics of a spinning
javelin.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
Justin,
The downward pitching motion is caused aerodynamically. The center of
gravity for the weapon is forwards, while its center of pressure is behind
it. The javelin will rotate on its center of gravity, but the air resistance
acts as a force pushing at the center of pressure. In other words, gravity
and inertial pull the front of the javelin down and forwards, while wind
pushes the back, well, back.
The gyroscopic forces involved are relatively insignificant. while they are
enough to counter mild fluctuations from the throw or small wind currents,
they are not great enough to have much noticeable effect on the orientation
versus flight direction. I suppose if you were to very carefully observe a
javelin flight, you may notice javelins thrown with the right hand twisting
just a little to the left, and vice versa. (gyroscopic progression)
Ryan Belscamper
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Update: June 2012
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