Physics Help! Newton's Laws!?

A Chinook salmon has a maximum underwater speed of 3.0 m/s, and can jump out of the water vertically with a speed of 6.5 m/s. A record salmon has a length of 1.5 m and a mass of 59 kg. When swimming upward at constant speed, and neglecting buoyancy, the fish experiences three forces: an upward force F exerted by the tail fin, the downward drag force of the water, and the downward force of gravity. As the fish leaves the surface of the water, however, it experiences a net upward force causing it to accelerate from 3.0 m/s to 6.5 m/s. Assuming that the drag force disappears as soon as the head of the fish breaks the surface and that F is exerted until 2/3 of the fish's length has left the water, determine the magnitude of F.

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  • 9 years ago
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    when the fish is in the water, we have the equation

    F - mg - D =0 where D is the drag of the water

    when the head breaks the water, D-> and we have that F - mg = ma

    we can calculate a from

    vf^2=v0^2 + 2ad

    vf=final velocity = 6.5m/s

    v0=initial velocity = 3m/s

    a=accel to be found

    d= 1 m

    how did we get that? we are told F is exerted until the fish is 2/3 out of the water. Since the fish is 1.5m in length, this means the fish still exerts the force F for a distance of 1 m

    so we have

    6.5^2 = 3^2 +2*a*1 => a = 16.62m/s/s

    go back to

    F - mg = ma

    F= m(g+a) = 59kg(9.8m/s/s+16.62m/s/s)

    F=1559N

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