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# Fisika Dasar I Per.21

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Fisika Dasar I Per.21

Fisika Dasar I Per.21

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## Fisika Dasar I Per.21Presentation Transcript

• Fisika Dasar I Umiatin, M.Si
• Jurusan Fisika
• Fakultas Matematika dan Ilmu Pengetahuan Alam
• GELOMBANG
• Pertemuan ke-21
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• Outline
• The vector product (cross product)
• TORKA
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 1. Sound Wave
• The most common example of longitudinal wave
• Divided into three categories :
• Audible ( frequency in the range of sensitivity of the human ear, eq : musical instrument, human voice)
• Infrasonic ( freq below audible range, eq : communicate between elephant )
• Ultrasonic (above audible, dog, medical imaging)
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 2. Speed of Sound Wave
• The speed of sound waves in a medium depends on the compressibility and density of the medium.
• If the medium is a liquid or a gas and has a bulk modulus B and density ρ , the speed of sound waves in that medium is :
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• Sound travels different speeds in different media. Sound typically travels faster in a solid that a liquid and faster in a liquid than a gas. The denser the medium, the faster sound will travel. The higher the temperature, the faster the particles of the medium will move and the faster the particles will carry the sound. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• In general, the speed of sound wave :
• Also depend on temperature :
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 3. Periodic Sound Wave
• If s ( x , t ) is the position of a small element relative to its equilibrium position,1 as :
• s max is the maximum position of the element relative to equilibrium. This is often called the displacement amplitude of the wave
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The Derivation
• From the definition of bulk modulus, the pressure variation in the gas is
• The element has a thickness ∆ x in the horizontal direction and a cross-sectional area
• A, so :
• As ∆ x approaches zero
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 4. Intensity of Periodic Sound Wave
• An oscillating piston transfers energy to the air in the tube, causing the element of air of width ∆ x and mass ∆ m to oscillate with an amplitude s max .
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• To evaluate the kinetic energy of this element of air, we need to know its speed.
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• As the sound wave moves through the air, this amount of energy passes by a given point during one period of oscillation. Hence, the rate of energy transfer is
• where v is the speed of sound in air.
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• We define the intensity I of a wave , or the power per unit area, to be the rate at which the energy being transported by the wave transfers through a unit area A perpendicular to the direction of travel of the wave
• In the present case, therefore, the intensity is
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The intensity of sound decreases as we move farther from the source. We identify an imaginary sphere of radius r centered on the source. When a source emits sound equally in all directions, we describe the result as a spherical wave. The average power, P av emitted by the source must be distributed uniformly over this spherical surface of area 4∏ r 2 . Hence, the wave intensity at a distance r from the source is
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The greater the intensity of sound the farther the sound will travel and the louder the sound will appear. Loudness is very closely related to intensity. Loudness is the human perception of the sound intensity. The unit for loudness is decibels . 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 5. Sound Level in Decibel
• Physical measurement of the strength of a sound Intensity of human ear is represented in logarithmic scale :
• Io: reference intensity / threshold (10 -12 W/m 2 ),
• I : intensity (W/m 2 )
• β : sound level (dB)
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• Loudness in Decibels 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The pitch of a sound wave is directly related to frequency . A high-pitched sound has a high frequency (a screaming girl). A low-pitched sound has a low frequency (a fog-horn). A healthy human ear can hear frequencies in the range of 20 Hz to 20,000 Hz . Humans cannot hear below 20 Hz. Sounds below this frequency are termed infrasonic . Sounds above 20,000 Hz are termed ultrasonic . Some animals, such as dogs, can hear frequencies in this range in which humans cannot hear. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 6. The Doppler Effect
• An observer O (the cyclist) moves with a speed v o toward a stationary point source S , the horn of a parked truck. The observer hears a frequency f’ that is greater than the source frequency .
• When the observer moves toward the source, the speed of the waves relative to the observer is v’= v + v o but the λ unchange
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• Because of :
• For observer moving forward :
• For observer moving away :
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• If the source in motion, the wave fronts heard by the observer are closer together than they would be if the source were not moving. As a result, the wavelength λ ’ measured by observer A is shorter than the wavelength
• λ of the source, the source moves at distance :v s .T = v s /f and the wavelength is shortened by this amount.
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The source moving forward :
• The source moving away :
• The general relationship for the observed frequency:
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• 7. Shock Wave
• (a)Shock wave produced when a source moves from So to Sn with a speed v s , which is greater than the wave speed v in the medium. The envelope of the wave fronts forms a cone whose apex half-angle is given by sin θ = v / v s . (b) A stroboscopic photograph of a bullet moving at supersonic speed through the hot air above a candle.
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The ratio v S / v is referred to as the Mach number , and the conical wave front produced when v S > v (supersonic speeds) is known as a shock wave .
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• The V -shaped bow wave of a boat is formed because the boat speed is greater than the speed of the water waves it generates.
• Analogous to a shock wave formed by an airplane traveling faster than sound. Jet airplanes traveling at supersonic speeds produce shock waves. The shock wave carries a great deal of energy concentrated on the surface of the cone. Such shock waves are unpleasant to hear and can cause damage to buildings when aircraft fly supersonically at low altitudes.
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• REFFERENCE
• SERWAYS
11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
• TERIMA KASIH 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |