The correct option is
D 120∘For a particle executing SHM, we can say that
x=Asin(ωt+ϕ)
Given two particles are executing SHM, we can write that
x1=Asin(δ1) and
x2=Asin(δ2)
Where
δ1 and
δ2 are the phase of two particles.
From the data given in the question, two particles pass one another going in opposite directions when their displacements are half of their amplitudes.
∴ x1=x2=A2, we get
For particle 1:
Assuming particle is moving away from mean position.
⇒A2=Asin(δ1)⇒sin(δ1)=12⇒δ1=π6
Similarly, For particle 2:
Assuming particle is moving towards mean position.
⇒A2=Asin(δ2)⇒sin(δ2)=12⇒δ2=5π6
Phase difference between the two particles is
δ=|δ1−δ2|=5π6−π6=2π3 or
120∘
Thus, option (d) is the correct answer.
Alternate solution :
Phasor diagram of the particles executing SHM is shown below.
From the diagram we can deduce that,
cosϕ=xA
When the particles are crossing each other at
x=A2 we get,
cosϕ=A2A⇒ϕ=cos−112⇒ϕ=π3
so, phase difference between the two particles is given by,
δ=2ϕ=2π3 or
120∘
Thus, option (d) is the correct answer.