The motion of a mass on a spring, with spring constant K is as shown in figure. The equation of motion is…

The motion of a mass on a spring, with spring constant K is as shown in figure.

The equation of motion is given by, x(t)=Asinωt+Bcosωt with ω=Km.
Suppose that at time t=0, the position of mass is x(0) and velocity v(0), then its displacement can also be represented as x(t)=Ccos(ωt-ϕ), where C and ϕ are 

  1. C=2v(0)2ω2+x(0)2, ϕ=tan-1v(0)x(0)ω
  2. C=2v(0)2ω2+x(0)2, ϕ=tan-1x(0)ω2v(0)
  3. C=v(0)2ω2+x(0)2, ϕ=tan-1x(0)ωv(0)
  4. C=v(0)2ω2+x(0)2, ϕ=tan-1v(0)x(0)ω

Solution

x=Asinωt+Bcosωt
v=dxdt=Aωcosωt-Bωsinωt
At t=0, x(0)=B
v(0)=Aω
x=Asinω+Bsinωt+90°

Anet=A2+B2
tanα=BAcotα=AB
x=A2+B2sin(ωt+α)
x=A2+B2cos(ωt-(90-α))

x=Ccos(ωt-ϕ)

C=A2+B2

C=[v(0)]2ω2+[x(0)]2
ϕ=90-α
tanα=cosα=AB

tanϕ=v(0)x(0)·ω
ϕ=tan-1v(0)x(0)ω

Asked in: JEE Main 2021 (22 Jul Shift 1)

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