Q77 / 90 · JEE Main 2021 (17 Mar Shift 1) · Physics

A boy is rolling a 0.5 kg ball on the frictionless floor with the speed of 20 m s-1. The ball gets deflected by an obstacle on the way. After deflection it moves with 5% of its initial kinetic energy. What is the speed of the ball now?

  • A
    19.0 m s-1
  • Correct answer
    B
    4.4 m s-1
  • C
    14.41 m s-1
  • D
    1.00 m s-1

Solution

Given, m=0.5 kg and u=20 m s-1

Initial kinetic energy ki=12mu2

=12×0.5×20×20=100 J

After deflection it moves with 5% of ki

∴kf=5100×ki⇒5100×100

⇒kf=5 J

Now, let the final speed be 'v'  m s-1, then:

kf=5=12mv2

⇒v2=20

⇒v=20=4.47 m s-1

Hence, the correct answer is option: 'B'.

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