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CBSE-XI-Physics

05: Newton's Laws of Motion

with Solutions - page 5
Qstn# iv-14 Prvs-QstnNext-Qstn
  • #14
    A pendulum bob of mass 50 g is suspended from the ceiling of an elevator. Find the tension in the string if the elevator (a) goes up with acceleration 1.2 m/s2, (b) goes up with deceleration 1.2 m/s2, (c) goes up with uniform velocity, (d) goes down with acceleration 1.2 m/s2, (e) goes down with deceleration 1.2 m/s2 and (f) goes down with uniform velocity.
    Ans : (a) When the elevator goes up with acceleration 1.2 m/s2:

    `` T=mg+ma``
    ⇒ T = 0.05 (9.8 + 1.2) = 0.55 N (b) Goes up with deceleration 1.2 m/s2 :

    `` T=mg+m\left(-a\right)=m\left(g-a\right)``
    ⇒ T = 0.05 (9.8 - 1.2) = 0.43 N (c) Goes up with uniform velocity:

    `` T=mg``
    ⇒ T = 0.05 × 9.8 = 0.49 N (d) Goes down with acceleration 1.2 m/s2 :

    `` T+ma=mg``
    `` \Rightarrow T=m\left(g-a\right)``
    ⇒ T = 0.05 (9.8 - 1.2) = 0.43 N (e) Goes down with deceleration 1.2 m/s2 :

    `` T+m\left(-a\right)=mg``
    `` \Rightarrow T=m\left(g+a\right)``
    ⇒ T = 0.05 (9.8 + 1.2) = 0.55 N (f) Goes down with uniform velocity:

    `` T=mg``
    ⇒ T = 0.05 × 9.8 = 0.49 N
    Page No 80:
  • #14-a
    goes up with acceleration 1.2 m/s2,
    Ans : When the elevator goes up with acceleration 1.2 m/s2:

    `` T=mg+ma``
    ⇒ T = 0.05 (9.8 + 1.2) = 0.55 N
  • #14-b
    goes up with deceleration 1.2 m/s2,
    Ans : Goes up with deceleration 1.2 m/s2 :

    `` T=mg+m\left(-a\right)=m\left(g-a\right)``
    ⇒ T = 0.05 (9.8 - 1.2) = 0.43 N
  • #14-c
    goes up with uniform velocity,
    Ans : Goes up with uniform velocity:

    `` T=mg``
    ⇒ T = 0.05 × 9.8 = 0.49 N
  • #14-d
    goes down with acceleration 1.2 m/s2,
    Ans : Goes down with acceleration 1.2 m/s2 :

    `` T+ma=mg``
    `` \Rightarrow T=m\left(g-a\right)``
    ⇒ T = 0.05 (9.8 - 1.2) = 0.43 N
  • #14-e
    goes down with deceleration 1.2 m/s2 and
    Ans : Goes down with deceleration 1.2 m/s2 :

    `` T+m\left(-a\right)=mg``
    `` \Rightarrow T=m\left(g+a\right)``
    ⇒ T = 0.05 (9.8 + 1.2) = 0.55 N
  • #14-f
    goes down with uniform velocity.
    Ans : Goes down with uniform velocity:

    `` T=mg``
    ⇒ T = 0.05 × 9.8 = 0.49 N
    Page No 80: