Answer:
0.66c
Explanation:
Use length contraction equation:
L = L₀ √(1 − (v²/c²))
where L is the contracted length,
L₀ is the length at 0 velocity,
v is the velocity,
and c is the speed of light.
900 = 1200 √(1 − (v²/c²))
3/4 = √(1 − (v²/c²))
9/16 = 1 − (v²/c²)
v²/c² = 7/16
v = ¼√7 c
v ≈ 0.66 c
Answer:
the source of sound moves towards an observe
Explanation:
The Doppler effect is related to waves such as sound or light. the effect causes an increase or decrease in the frequency of sound light or other waves when the souces either move towards or away from the observer. For example the siren of the train to a person on the platform, the redshift seen by astronomers.
Therefore, The Doppler shift can be observed when the source of sound moves towards an observer From a place closer to the observer than the last wave's crest, each consecutive wave crest is sent. Each wave therefore, takes a little less time than the preceding wave to reach the observer.
We have that the spring constant is mathematically given as

Generally, the equation for angular velocity is mathematically given by

Where
k=spring constant
And

Therefore

Hence giving spring constant k

Generally
Mass of earth 
Period for on complete resolution of Earth around the Sun


Therefore


In conclusion
The effective spring constant of this simple harmonic motion is

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Answer:
3 fans per 15 A circuit
Explanation:
From the question and the data given, the light load let fan would have been
(60 * 4)/120 = 240/120 = 2 A.
Next, we add the current of the fan motor to it, so,
2 A + 1.8 A = 3.8 A.
Since the devices are continuos duty and the circuit current must be limited to 80%, then the Breaker load max would be
0.8 * 15 A = 12 A.
Now, we can get the number if fans, which will be
12 A/ 3.8 A = 3.16 fans, or approximately, 3 fans per 15 A circuit.