Page:Popular Science Monthly Volume 75.djvu/35

Rh the handle bar is turned, and although this gyrostatic action helps to maintain the equilibrium of the rider, it is very small in its effect as compared with the linear momentum of the rider and bicycle frame.

Fig. 24 is a top view of an axle and pair of drive wheels of a locomotive rounding a railway curve. The arrow S in the vector diagram represents the spin-momentum of the axle and drivers at a given instant, S' represents the spin-momentum at a later instant, ΔS represents the increment of spin-momentum, and T represents the torque which must act upon the axle because of its precession. To exert this torque the outer rail must push up with an excessive force against the outer driver, or, in other words, the outer driver must be forced downwards against the outer rail with more force than that which is due to the locomotive alone as it is rounding a curve. That is to say, the gyrostatic action of the drivers of a locomotive exaggerates the excess of pressure on the outer rail while the locomotive is rounding a curve.

The most familiar type of boomerang is a pair of crossed sticks twisted very slightly at the ends like the vanes of a windmill. This type of boomerang, which we will call the propeller-wheel type, is essentially similar in its action to the boomerang of the native Australians. The boomerang is thrown through the air with a spinning motion about an axis at right angles to the plane of the crossed pair of sticks, and the peculiar flight of the boomerang is due to the forces exerted upon the boomerang by the air.

Forces are exerted upon the moving boomerang very much as if it were a disk traveling approximately edgewise through the air and forces are exerted upon the boomerang by virtue of its propeller-wheel shape and because of its combined spinning and edgewise motion. The effects of these two sets of forces will be described separately and their combined action will then be made use of in explaining the actual motion of the boomerang.

A disk moving approximately edgewise through the air is in an unstable condition, if the disk starts to glance to one side or the other the air exerts a turning force or torque upon it which tends to turn