2 1. INTRODUCTION
1.1.2 NEWTON’S CRADLE
e term Newton’s cradle is used mainly for terminological convenience and for commercial iden-
tification in toy shops. A commonly available version is that shown in Figure 1.2 in which five
identical spherical metal balls are similarly suspended by wires attached to both sides. e main
technical interest of this toy is in its use to demonstrate the impact of multiple particles. For ex-
ample, if one pulls two spherical balls on the left-hand side (lhs) of the cradle and releases them
simultaneously so that they hit the three remaining stationary balls. It is interesting to observe
that only two of the three stationary balls will fly to the right-hand side (rhs). If one pulls three
balls to the lhs and releases them so that they hit the two remaining balls. It is observed that
only the three balls will fly to the rhs.
Figure 1.2: Newton’s cradle.
1.1.3 SIMPLE TOY ROTOR
e simple rotor can easily be found in many toy shops. e bottom view of one version is shown
in Figure 1.3a, while the sketch of side view of the toy rotor on a glass panel is included in Fig-
ure 1.3b where the counter-clockwise black arrows denote the direction of the applied transient
torque (produced by the twisting action from the holding fingers that are quickly removed). e
transient torque enables the rotor to rotate in the counter-clockwise direction for 2 or 3 s (the
duration of this rotation depends on the magnitude of the twisting action) before it stops mo-
mentarily for a fraction of a second. en the rotor rotates freely in the clockwise direction (that
is, the direction of rotation now is opposite to that of the originally applied transient torque).
is rotation is indicated by the two blue arrows in the figure. e interesting point to note in
this system is that the rotor rotates in one direction and stops momentarily before it rotates freely
in the opposite direction. is example demonstrates the importance of the distribution of mass
moment of inertia on stability in the design of propellers in ships, aircrafts, and helicopters.
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