[Illustration: FIG. 90.]
641. The centre of oscillation in a body free to rotate about a fixed
axis is identical with another remarkable point, called the _centre
of percussion_. We proceed to examine some of the properties of a
body thus suspended with reference to the effects of a blow. For the
purposes of these experiments the method of suspension by edges is
however quite unsuited.
642. We shall first use a rod suspended from a pin about which the
rod can rotate. A B, Fig. 90, is a pine rod 48" × 1" × 1", free to
turn round B. Suppose this rod be hanging vertically at rest. I take a
stick in my hand, and, giving the rod a blow, an impulsive shock will
instantly be communicated to the pin at B; but the actual effect upon
B will be very different according to the position at which the blow
is given. If I strike the upper part of the rod at D, the action of A
B upon the pin is a pressure to the left. If I strike the lower part
at A, the pressure is to the right. But if I strike the point C, which
is distant from B by two-thirds of the length of the rod, there is no
pressure upon the pin. Concisely, for a blow below C, the pressure is
to the right; for one above C, it is to the left; for one at C it is
nothing.
643. We can easily verify this by holding one extremity of a rod
between the finger and thumb of the left hand, and striking it in
different places with a stick held in the right hand; the pressure of
the rod, when struck, will be so felt that the circumstances already
stated can be verified.
[Illustration: FIG. 91.]
644. A more visible way of investigating the subject is shown in Fig.
91. F B is a rod of wood, suspended from a beam by the string F G. A
piece of paper is fastened to the rod at F by means of a small slip of
wood clamped firmly to the rod; the other ends of this piece of paper
are similarly clamped at P and Q.
645. When the rod receives a blow on the right-hand side at A, we
find that the piece of paper is broken across at E, because the end
F has been driven by the blow towards Q, and consequently caused the
fracture of the paper at a place, E, where it had been specially
narrowed. I remove the pieces of paper, and replace them by a new piece
precisely similar. I now strike the rod at B,—a smart tap is all that
is necessary,—and the piece of paper breaks at D. Finally replacing the
pieces of paper by a third piece, I find that when I give the rod a tap
(not a violent blow) at C, neither D nor E are broken.
646. This point C, where the rod can receive a blow without producing
a shock upon the axis of suspension is the _centre of percussion_. We
see, from its being two-thirds of the length of the rod distant from
F, that it is identical with the centre of oscillation of the rod, if
vibrating about knife-edges at F. It is true in general, whatever be
the shape of the body, that the centre of oscillation is identical with
the centre of percussion.
647. The principle embodied in the property of the centre of percussion
has many practical applications. Every cricketer well knows that there
is a part of his bat from which the ball flies without giving his hands
any unpleasant feeling. The explanation is simple. The bat is a body
suspended from the hands of the batman; and if the ball be struck with
the centre of percussion of the bat, there is no shock experienced.
The centre of percussion in a hammer lies in its head, consequently a
nail can receive a violent blow with perfect comfort to the hand which
holds the handle.