Phase of workdone by water jet striking flat stationary plate normal to jet

Q.  Consider the below diagram of jet of water striking flat stationary plate normal to jet. What will be the phase of workdone on the plate?
- Published on 16 Sep 15
Jet-of-water-striking-flat-stationary-plate-normal-to-jet.png


a. negative
b. positive
c. zero
d. unpredictable

ANSWER: zero

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    Discussion

  • iba   -Posted on 16 Jun 20
    What is the formula of work done here?
  • Pankaj   -Posted on 06 Oct 15
    The given diagram is of jet of water striking flat stationary plate normal to jet.

    Consider,

    V = Velocity of jet, m/s

    A = Cross-sectional area of jet, m2

    ρ = density of fluid, kg/m3

    As the plate is fixed, its velocity is zero.

    When jet strikes the the plate, its momentum is destroyed and final velocity becomes zero. The force acting on the plate in the direction normal to the plate can be determined by impulse momentum equation,

    F = ṁ (V1 – V2)

    Where,
    ṁ = ρ Q

    = ρ A V

    Therefore,

    F = ρ A V (V – 0)

    F = ρ A V2

    As the plate is stationary and no change in its velocity, the workdone on plate is zero.

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