http://www.ck12.org Chapter 10. Periodic Motion
corner from you. Light that passes through a small opening can illuminate an entire room. It is more difficult to
detect the diffraction of light than that of sound. You might try having a thin object cast a shadow on a very bright
day. If you look at the edges of the shadow, you may be able to see that edge is not sharp. The lack of sharpness
is due to the light bending about the object. All waves diffract around objects and spread out through any openings
available to them as inFigure10.20. There is a relationship between the size of the opening and the nature of the
wave that determines the degree to which the wave spreads that will be discussed later.
FIGURE 10.20
Notice in the leftmost diagram the possi-
bility of a “shadow zone,” an area behind
the obstacle where the waves are less
pronounced.- The projection of uniform circular motion onto a straight line is calledsimple harmonic motion(SHM); it is
the simplest form of periodic motion. - An object performing SHM must have a restoring force upon it that seeks to return it to its equilibrium position
which is directly proportional to the object’s displacement; that isF=−kxwhere the negative sign indicates
that the restoring force and displacement are oppositely directed andkis a constant of proportionality. - The periodTfor a massmon a spring isT= 2 π
√m
kwhere the spring is constant isk.- The frequency isf=T^1 orf= 21 π
√
k
m- The periodTof a simple pendulum of lengthLisT= 2 π
√
L
gwhere the gravitational acceleration isg.
(a) Waves transmit energy but not mass.- Longitudinal waves occur when vibrations are parallel to wave direction.
- Transverse waves occur when vibrations are perpendicular to the wave direction.
- Reflection occurs when waves bounce off one medium back into the medium they originated from.
- Refraction occurs when waves bend entering one medium from the other.
- Diffraction occurs when waves spread out and when waves bend behind objects.