Physics 11

Name: __________________________________

Sound  ______________ waves of pressure that stimulate our ear drums

 The speed of sound depends on its ___________ (more dense  __________)

 Pitch is determined by _________________

 Volume is determined by _________________

The Doppler Effect Imagine a water bug standing on the surface of a pond, bouncing up and down.

Now imagine that bug skimming along the surface of the pond. The waves in back:

The waves in front:

The Doppler Effect and Red Shift 

When we look at other galaxies we notice that their spectra (colours) are shifted towards ________.



This is a result of a Doppler shift of the EM Radiation towards a _____________ wavelength.



Therefore the galaxies must be moving __________ __________ us.



And so the universe must be _______________.

Physics 11

Name: __________________________________ Doppler Effect and Sonic Booms

A stationary sound source emits circular waves, as shown below:

Consider now what happens if the source is moving to the right, as shown below. The sound waves do not travel any faster in front of the source than they do behind the source, since the speed of a sound wave is determined only by the medium; however, during the time between successive waves the source has moved toward the observer and has also gotten closer to the waves produced in front of it. The result is that waves in front of the source are being produced closer together, and waves behind the source are being produced farther apart. This reduces the wavelength for the observer. Since f = v /λ , this means that the observer will hear a higher frequency. Behind the source, the larger wavelength will result in a lower frequency. This is known as the Doppler Effect. Consider now a source, such as a jet, that is travelling at the speed of sound. In this situation, the jet is keeping up with the wavefronts in front of it and successive waves are being produced on top of previous ones, creating a huge sound wave. Remember that sound waves are compressed air molecules, so this represents a massive compression of air molecules. This “wall of air” represents the sound barrier that an object must break through in order to go faster than the speed of sound.

Suppose now that the jet breaks through this barrier by going faster than the speed of sound; since it is now going faster than the sound waves, the jet will stay in front of the sound waves, as shown in Figure 20. Notice the overlapping waves behind the jet; these create constructive interference which results in a loud cone of noise being heard. This is known as the sonic boom. Notice in the figure below that the sonic boom trails the jet, growing larger but weaker as it expands. A person on the ground would hear the sonic boom after the jet has passed by.