Doppler effect and signs of the equation

AI Thread Summary
The discussion focuses on applying the Doppler effect to determine the frequency difference heard by a scooter driver as they approach and then pass a police car with a siren. The scooter travels at 10 m/s while the police car moves at 50 m/s in the opposite direction, with the siren's frequency at 700 Hz when stationary. Participants clarify the use of the Doppler effect equation, emphasizing the correct application of velocities of the source and listener in relation to the speed of sound. They highlight that when the source moves toward the listener, the frequency increases, necessitating specific mathematical adjustments in the equation. Understanding these principles is crucial for accurately calculating the frequency difference experienced by the scooter driver.
RUphysics3
Messages
24
Reaction score
0

Homework Statement


A motor scooter moving at 10 m/s approaches and passes a police car moving at 50 m/s in the opposite direction. The frequency of the siren on the police car is 700 hz when the car stands still. the difference in frequency heard by the driver of the scooter between when the two vehicles are approaching each other and when the two vehicles are separating from each other is closest to..

Homework Equations


doppler effect equation

The Attempt at a Solution


I know I must find the frequency at one point and subtract it from the other to find the difference, but in the doppler equation I never know when the velocity of the source/listener must be subtracted/added to the speed of sound
 
Last edited:
Physics news on Phys.org
RUphysics3 said:

Homework Statement


A motor scooter moving at 10 m/s approaches and passes a police car moving at 50 m/s in the opposite direction. The frequency of the siren on the police car is 700 hz when the car stands still. the difference in frequency heard by the driver of the scooter between when the two vehicles are approaching each other and when the two vehicles are separating from each other is closest to..

Homework Equations


Doppler effect equation

The Attempt at a Solution


I know I must find the frequency at one point and subtract it from the other to find the difference, but in the doppler equation I never know when the velocity of the source/listener must be subtracted/added to the speed of light.
I hope you mean "Speed of Sound" not of light.
 
  • Like
Likes RUphysics3
SammyS said:
I hope you mean "Speed of Sound" not of light.
i do indeed lol
 
RUphysics3 said:
I know I must find the frequency at one point and subtract it from the other to find the difference, but in the doppler equation I never know when the velocity of the source/listener must be subtracted/added to the speed of sound
You know that is always a matter of multiplying or dividing by 1+v/c or 1-v/c, where c is the speed of sound and v is one of the other velocities.
You also know that when the source is moving towards the receiver the frequency will increase, so for that case it must be either multiply by 1+v/c or divide by 1-v/c.
A simple thought experiment will tell you which. Suppose the source is moving at the speed of sound. What will happen to the wavefronts? What frequency will that seem like to the receiver when they meet?
 

Thread 'Struggling to understand the concept of this question (ice cube in water optics question)'

TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
View full post »
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
View full post »

Similar threads

Range of frequency heard by observer related to Doppler effect
Replies
8
Views
1K
Doppler effect and beat frequency
Replies
3
Views
2K
How do you calculate wind speed using the Doppler effect?
Replies
4
Views
3K
Calculating frequencies with the Doppler Effect
Replies
1
Views
3K
Blood flow velocity via Doppler effect
Replies
1
Views
2K
Beats calculation/Doppler Effect
Replies
1
Views
2K
Doppler effect train frequency question
Replies
4
Views
2K
How to Solve a Doppler Effect Problem with a Moving Vehicle and Siren at Rest
Replies
1
Views
1K
Spring and Microphone (Doppler Effect...?)
Replies
7
Views
3K
Doppler Effect: Velocity of a train
Replies
4
Views
2K

Hot Threads

Recent Insights

Back
Top