Section 8.1 Vibrations Learning Goal: By the end of today, I will be familiar with the following concepts: Vibrations and Mechanical waves The cyclical motion about an equilibrium point is called a vibration. This transfer of energy through a material by particle vibration is called a mechanical wave. The material through which a mechanical wave travels is called a medium. A medium can be a solid, a liquid, or a gas. Demo Newton's Cradle Tuning Forks Dec 4 2:58 PM Dec 4 2:58 PM 1
Key Words Video motion of a drum head Dec 4 3:04 PM Dec 4 3:08 PM 2
Particle Behaviour in Different Media Molecules are always in motion because of thermal energy. However, thermal motion motion resulting from thermal energy is random and does not produce a transfer of energy in the form of a mechanical wave. Instead, the medium has to be disturbed by a vibration to set up a mechanical wave. All vibrations need a medium to transfer waves. A medium s effectiveness at transmitting vibrations varies, depending on its molecular and mechanical structure, its density, and even its temperature. In a solid medium, the atoms are held securely in a crystal formation by strong intermolecular forces. Therefore, they can only vibrate slightly as the disturbance passes through the medium. If the medium returns to its original shape after the disturbance, the medium is said to be elastic. Most solid media have this property even very rigid media, such as steel. In general, rigid materials transfer mechanical waves more efficiently than less rigid materials. Th us, mechanical waves in rigid materials last longer, go faster, and go farther than they do in less rigid media. Liquids and gases are classified as fluids because they are materials that can flow. In liquids, the molecules are not in a crystal formation but are still very much in contact. So liquids are very effective transmitters of sound. For example, sound travels almost five times as fast and much farther in water than in air. The individual molecules in a gas are much farther apart than they are in liquids and solids. Consequently, gas is the least dense state of matter. Gases rely on translational molecular motion, or straight line motion, to transfer vibrations. Video bunker bomb With their lower density, gases are less effective than solids and liquids at transmitting vibrations. How well a gas transmits a vibration also depends on the gas s temperature and density Conversely, the less rigid a medium, the less efficient it is at transferring a vibration. A less rigid material, such as a pillow, disperses more energy through absorption, so a vibration weakens quickly. The speed and distance that a wave can travel are therefore reduced. Self Test: tap a coin in the desktop very gently, almost to the point where you can't hear it. Then place your ear on the desk and repeat; move as far away as the surface will allow. Dec 4 3:40 PM Dec 4 3:46 PM 3
Section 8.2 Mechanical Waves Mechanical waves can be classified according to the direction of the particle motion compared to the direction of the wave motion. There are two basic types of mechanical waves: transverse waves and longitudinal waves. Dec 4 3:50 PM Dec 4 3:50 PM 4
Transverse (shear) Waves A transverse wave describes a wave in which the particles vibrate perpendicular to the direction of the flow of energy. Longitudinal Waves A longitudinal wave describes a wave in which the particles vibrate in the same direction as the energy flow compression Energy Guitar strings and water waves are examples of Transverse Waves. A pulse is a single wave or single disturbance. Demo: Large Slinky on floor Dec 4 3:52 PM Dec 4 3:50 PM 5
Sound A vibrating object can produce longitudinal waves in a part of the medium that surrounds it. If these waves have properties that make them detectable to the human ear, they are called sound. The energy transferred through successive compressions and rarefactions of a sound wave causes vibrations in our ears that the brain interprets as sound. (Human hearing is discussed further in Chapter 10.) Sound is also transmitted through liquids and solids. Our ears are less suited to detecting sound waves in liquids and solids, however. In fluids, sound is transmitted as a longitudinal wave. In solids, sound can be transmitted as either a transverse wave or a longitudinal wave. Video Bomb Blast Video Sonic Boom Dec 4 3:08 PM Dec 4 4:02 PM 6
Dec 4 4:07 PM Dec 4 4:09 PM 7