Q38 / 54 · JEE Advanced 2018 (Paper 1) · Physics

Consider a body of mass 1.0 kg at rest at the origin at time t=0. A force F→=αti^+βj^ is applied on the body, where α=1.0Ns-1and β=1.0N . The torque acting on the body about the origin at time =1.0 s is τ→ . Which of the following statements is (are) true?

  • Correct answer
    A
    τ→=13 Nm
  • B
    The torque τ→ is in the direction of the unit vector +k→
  • Correct answer
    C
    The velocity of the body at t=1 s is v→=12i^+2j^ m s-1
  • D
    The magnitude of displacement of the body at t=1 s is16m

Solution

F→=αti^+βj^ at t=0, v=0,r→=0→
α=1, β=1
F→=ti^+j^
mdv→dt=ti^+j^
On integrating
mv→=t22i^+tj^ m=1kg
dv→dtt22i^+tj^ r→=0→ at t=0
On integrating
r→=t36i^+t22j^
τ→=-13k^
v→=t22i^+tj^
At t=1 v→=12i^+j^=12i^+2j^m/sec
At t=1 s→=r1→-r0→
=16i^+12j^-0→
s→=16i^+12j^
s→=162+122 = 106m

Hence, the correct answer is options: 'A', 'C'.

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