The number of times something happens in one second is its "frequency".
when observer and source moves relative to each other then the frequency received by the observer is different from the real frequency
This apparent change in frequency due to relative motion is known as Doppler's effect.
Here we know that

here we know that
= real frequency
v = speed of sound
= speed of observer
= speed of source
so this is known as Doppler's Effect
Answer:
0.6983 m/s
Explanation:
k = spring constant of the spring = 0.4 N/m
L₀ = Initial length = 11 cm = 0.11 m
L = Final length = 27 cm = 0.27 m
x = stretch in the spring = L - L₀ = 0.27 - 0.11 = 0.16 m
m = mass of the mass attached = 0.021 kg
v = speed of the mass
Using conservation of energy
Kinetic energy of mass = Spring potential energy
(0.5) m v² = (0.5) k x²
m v² = k x²
(0.021) v² = (0.4) (0.16)²
v = 0.6983 m/s
b.....................................................