Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Waves and sound → Sound characteristics ↓


Doppler effect in sound


The Doppler effect is a fascinating phenomenon that affects the characteristics of sound waves. Named after Austrian physicist Christian Doppler, this effect describes how sound frequency changes relative to motion between the source of the sound and the observer. In this explanation, we'll cover the basics of sound waves, how the Doppler effect works, and provide examples and visuals to make it easier to understand this important concept in physics.

The basics of sound waves

Sound is a type of energy that travels through air (or other medium) in the form of waves. These are called sound waves, and they are created by vibrating objects. These waves are usually longitudinal, meaning that the displacement of the medium is in the same direction as the direction of wave propagation.

Sound waves have several characteristics:

  • Frequency: It is the number of waves passing a fixed point per second. The unit of frequency is Hertz (Hz).
  • Wavelength: It is the distance between successive crests of a wave.
  • Amplitude: This is the height of the wave and it determines the loudness or volume of the sound.
  • Speed: The speed of sound depends on the medium through which it is traveling. In air at room temperature, it is about 343 meters per second.

The relation between the speed ((v)), frequency ((f)), and wavelength ((lambda)) of a sound wave is given by the formula:

v = f times lambda

Explanation of the Doppler effect

The Doppler effect describes how the frequency of a wave changes when the source and observer are moving relative to each other. When dealing with sound, if the sound source is moving toward the observer, the observer perceives a higher frequency (the sound appears to be in a higher pitch). Conversely, if the sound source is moving away, the observer perceives a lower frequency (the sound appears to be in a lower pitch).

Let's take a closer look at this with an example:

An example of a moving car

Imagine a car moving towards you honking its horn. As the car approaches you, the sound waves get compressed, increasing the intensity of the sound. As it moves further away, the sound waves get stretched, decreasing the intensity of the sound. This change in the intensity of sound as the car moves forward is the action of the Doppler effect.

car

In the illustration above, the blue line represents the sound waves that are compressed as the car moves toward you, while the red line represents the longer waves that are produced as the car moves away.

Mathematics of the Doppler effect

The formula for observed frequency ((f')) due to Doppler effect when the source and observer are in motion is given by:

f' = frac{v + v_o}{v + v_s} times f
  • (f') = observed frequency
  • (v) = speed of sound in the medium
  • (v_o) = speed of observer (positive when moving towards the source)
  • (v_s) = speed of the source (positive when moving away from the observer)
  • (f) = actual frequency emitted by the source

Next example - ambulance siren

Imagine an ambulance racing towards you with a loud siren. As it gets closer to you, you hear the siren getting louder. As it moves away, the sound decreases. This is due to changes in the frequency of the sound waves reaching your ears, caused by the Doppler effect.

Ambulance

In this illustration, the principle is similar to that of a moving car. The pitch perceived changes due to the compression and stretching of the sound waves.

Everyday examples of the Doppler effect

The Doppler effect is not just limited to moving vehicles; it exists in many everyday situations. Here are some examples:

Weather radar systems

Weather radars use the Doppler effect to measure wind speed. They send out radio waves and then measure how those waves bounce back from raindrops moving relative to the radar. The frequency change in the returned signal helps determine wind speed and direction.

Speed guns

Police use radar guns to measure the speed of vehicles. This device sends radio waves towards a moving vehicle and calculates its speed by detecting changes in the frequency of the reflected waves.

Astronomy

Astronomers use the Doppler effect to estimate the speed and movement of stars and galaxies. It is also used to support the theory that the universe is expanding by looking at the redshift of distant galaxies.

Conclusion

The Doppler effect is a fundamental concept in the study of waves and sound. It can be used in many fields beyond simple sound waves, including technology, meteorology, and astronomy. To understand it, it is necessary to recognize how the relative motion between the wave source and the observer can affect the frequency and wavelength of the detected waves.

By analysing the processes involved and looking at practical examples, such as moving vehicles and their changing sounds, we can understand the role of the Doppler effect in our everyday experiences and technological progress.


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Doppler effect in sound

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