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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 19

Lesson 7: The Doppler effect

# When the source and the wave move at the same velocity

What happens when both the listener and the source are moving together? In this exploration, you'll uncover what happens when the velocity of a wave source matches the wave's speed. You'll delve into the effects on observed frequency and period, leading to a deeper understanding of phenomena like sonic booms. Remember, this discussion applies to any wave, not just sound. Created by Sal Khan.

## Want to join the conversation?

• I didn't get how exactly will the observer not hear anything till the source passes him.
• It is because the object is traveling at the same speed as the sound. So as the object travels it is moving along with the sound so it arrives at the same time as the sound.
• Are shock waves related to sonic boom?
• the definition of a sonic boom is the boom that you get once an object breaks through the sound barrier. basically, there's literally a barrier of sound that you need to break through. when a plane is going through the air, it starts to pile up waves up sound the faster it goes, so, later, there's a bunch of sound waves that is in front of the plane. then, the plane needs a big boost of speed to break through that barrier of sound, and when it does, you get something called the sonic boom. then, when the plane breaks the sound barrier, it leaves a trail of sound behind it and makes a v line that follows it and it basically is going faster than sound. So, when a plane breaks the sound barrier above you, it's gonna take some time to hear the sonic boom, then, if you run fast enough in ahead of the plane, you're going to hear the sonic boom again. This is basically sort of adding to what teodoro said. I hope this answered your question.
• What would happen if the source was moving faster than the wave? I substituted the values, and came up with a negative period. What does that mean?
• can the velocity of source be greater than velocity of wave? why?
(1 vote)
• Have you ever been on a fast moving boat? It travels faster than the waves it makes, right? You don't see any waves moving out ahead from the front of the boat. Instead you get an interesting wake pattern behind the boat.
• is there a video for source moving at a greater velocity than the wave?
• No there's not. But why don't you try to plug in the values on your own in the equations? ;)
• Are shock waves made by planes same as made by explosions? I'm confused.
• A shock wave is shock wave, doesn't really matter how it's made. A sonic boom sounds like an explosion.
• Doesn't the velocity of the source add to the velocity of the wave if the source is already moving when it emits the wave?