Why does a spear or a javelin turn in the air?

Question

In videos of the javelin throw, the throwers consistently seem to grip the spear at the center of mass and launch it by imparting it an initial velocity. What causes the javelin to turn around an axis passing through the center of mass if there is no net torque, since there is no force with a non-zero moment w.r.t. the center of mass?

Answer 1

From the wikipedia article about Javelin throw

In 1986 the technical committee of the International Association of Athletics Federations introduced new specification for the standard competition javelin.

Among other things it was specified that the center of mass should lie forward of the center of air resistance. This new specification gives a "nose heavy" javelin.

The previous standard competition javelin was designed to be balanced. When thrown with perfect balance the javelin would keep climbing for a long time.

More and more often the javelin would hit the ground while parallel to the ground, and would continue to slide for a distance. The javelin throw event became rather dangerous; sometimes the sliding javelin would slide all the way onto the running track.

Consequences of the change: I remember reading an interview with a javelin thrower, a couple of years after that change. He said it had shifted the event from highly technical to more muscle dominated.

The balanced javelin was very unforgiving; you had to throw it just right, otherwise the throw was a dud. The new javelin tends to self-straighten in flight, so the throw no longer needs to be perfect.

Answer 2

Throwing a spear straight without its tip falling or rising and hitting a target with it requires that the spear possess no angular velocity about its center of mass when released. This is a nontrivial task that takes a lot of practice.

The spear thrower has to move his or her arm through an shallow arc while throwing the spear. To make the spear fly straight without rotating, the thrower has to work his or her wrist and fingers in just such a manner as to not impart any net torque to the spear for the whole time they are accelerating the spear through that arc.

This is a hard task because to accelerate the spear requires a firm grip on the shaft at the same time the thrower is trying to cancel the arc. And on top of that, they also have to aim the spear so it will hit its intended target.

All this presupposes that the thrower is grasping the spear at its exact midpoint so at the moment of release there is no gravitational torque acting on it. This is yet another difficult task.

Answer 3

There may be no initial torque, but the spear cleaves the air to pass through. Air is composed of molecules that act with Newton's third law. Each molecule displaced contributes to the friction the spear has on its path. As a completely symmetric spear is very difficult to achieve, and unnecessary for the function of a spear, torque appears.

Answer 4

Other good answers, but if you've ever used model rockets, they had to pass the "swing test" to make sure they would stay nose-forward. That's why they had fins at the tail - so the center of gravity is forward of the center of aerodynamic force.

The same thing applies in airplanes. They are normally nose-heavy. This gives them stability. A good way to court an accident is to put too much weight in the rear. Even more, and it's hard to hold the nose down. They can even slide backwards, all the way to the ground.

Same goes in automobiles. When sliding sideways they should tend to straighten out. This is called "understeer". Rear-wheel drive cars that spin their wheels exhibit "oversteer" and they are a challenge to keep from spinning out.

Even shopping carts do it. The casters are on the front. Shove one backwards, and it immediately swaps ends.