Question

In the Doppler Effect, the observed frequency of a wave can be shifted to higher or lower frequencies, depending on how the source and observer are moving relative to each other. After reading the section in text about the Doppler Effect, evaluate whether the observed frequency would be higher, lower or the same as the source frequency in each of the following situations. (choices are higher, lower, same)
Observer is moving towards the source and the source is stationary
Source is moving towards the observer and the observer is stationary
Observer is moving away from the source and the source is stationary
Observer and source are both stationary
Source is moving away from the observer and the observer is stationary

Now how about these situations? (choices are higher, lower, same)
Observer and source each moving away from one another
Observer and source each moving due West at the same speed
Observer and source each moving towards one another

Answer 1

Answer:

1. Higher

2. Higher.

3. Lower.

4. The frequency is the same as the source.

5. Lower

Part B

1. Lower

2. Same

3. Higher

Explanation:

To explain this let's use the general equation for the observed frequency for doppler effect

f = fs×(V + Vo)/(V + Vs)

f = observed frequency

fs = frequency of source

V = velocity of sound in the medium (air)

Vs = velocity of the sound source

Vo = velocity of observer

Guide rules to use the equation above:

When observer is moving towards the source Vo is positve but negative when observer is moving away from the source

When source is moving towards the observer Vs is negative but poistive when source is moving away from the observer.

Part A

1. In this case Vo is positive and the numerator in the equation increases. Therefore the frequency heard by the observer is higher

2. In this case Vs is negative as the source is moving towards the observer and the denominator decreases. This causes the fraction in the equation to be greater than one and as a result the frequency is also higher

3. In this case Vo is negative as the observer is moving away from the source and the numerator decreases. This causes the fraction in the equation to be lesser than one and as a result the frequency is lower.

4. In this case Vs and Vo are both zero and as a reult the fraction in the equation reduces to 1. Therefore frequency is the same.

5. In this case Vs positive as the observer is moving away from the source and the denominator increases. This causes the fraction in the equation to be lesser than one and as a result the frequency is lower.

Part B

1. In this case Vs is positive and Vo is negative. This results in a fraction less than 1 and as a result the frequency observed is less than 1.

2. Their speeds relative to each other is constant and as a result the frequency observed is the same as that of the source

3. There speeds relative to each other increases and observed frquency is greater.