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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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