# 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.

Relevance

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=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