Derivative of F(t): Solving Doppler Shift Problem

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In summary, the conversation discusses a problem involving Doppler shifts as frequency changes with time and the task of finding F'(t). The attempt at a solution involves expanding F(t) and using the quotient and chain rules, but the specific steps used are incorrect. The person also mentions forgetting their calculus knowledge and asks for help.
  • #1
chlorine
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Homework Statement


This problem has to do with Doppler shifts as frequency changes with time.
F(t)=fo(1-(v^2*T)/(c*sqrt(v^2*T^2+b^2))
T=t-To
I want to find F'(t) .

Homework Equations


See #1.


The Attempt at a Solution


I expand F(t) to be
F(t)=f0-(fo*v/c)*(1+(b/(vT))^2)^1/2
and again...
F(t)=f0-(fo*v/c)*(1+b^2*v^-2*T^-2)^1/2

After this, I forgot all of my calculus I've learned couple years ago. :(
Please help me remember what to do.
I know you do exponent on the outside and then work inside... dot dot dot.

Thanks.
 
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  • #2
chlorine said:

Homework Statement


This problem has to do with Doppler shifts as frequency changes with time.
F(t)=fo(1-(v^2*T)/(c*sqrt(v^2*T^2+b^2))
T=t-To
I want to find F'(t) .

Homework Equations


See #1.


The Attempt at a Solution


I expand F(t) to be
F(t)=f0-(fo*v/c)*(1+(b/(vT))^2)^1/2
The above is incorrect for a number of reasons. I don't know what you did to get this. Don't bother multiplying by f0.
chlorine said:
and again...
F(t)=f0-(fo*v/c)*(1+b^2*v^-2*T^-2)^1/2

After this, I forgot all of my calculus I've learned couple years ago. :(
Please help me remember what to do.
I know you do exponent on the outside and then work inside... dot dot dot.

Thanks.

You need to use the quotient rule first, and you will need the chain rule to get the derivatives that arise in the quotient rule.
 

1. How is the derivative of F(t) used to solve a Doppler shift problem?

The derivative of F(t) is used to calculate the rate of change of the frequency of a wave as it moves towards or away from an observer. This is important in solving Doppler shift problems because it helps to determine the shift in frequency of a wave due to the relative motion between the source of the wave and the observer.

2. What is the formula for calculating the derivative of F(t) in a Doppler shift problem?

The formula for calculating the derivative of F(t) is given by: dF(t)/dt = -v/c * F(t), where v is the velocity of the source or observer and c is the speed of the wave.

3. Can the derivative of F(t) be used for both sound and light waves in a Doppler shift problem?

Yes, the derivative of F(t) can be used for both sound and light waves in a Doppler shift problem. However, the value of c will differ for sound and light waves as the speed of sound is much slower than the speed of light.

4. How does the direction of motion affect the derivative of F(t) in a Doppler shift problem?

The direction of motion affects the sign of the derivative of F(t). When the source or observer is moving towards each other, the derivative will have a positive sign. When they are moving away from each other, the derivative will have a negative sign.

5. Can the derivative of F(t) be used to calculate the change in wavelength in a Doppler shift problem?

Yes, the derivative of F(t) can be used to calculate the change in wavelength in a Doppler shift problem. This is because wavelength is inversely proportional to frequency, so as the frequency changes, the wavelength will also change by the same proportion.

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