29 Newtons
Centripetal force is modeled by the equation:
F = MV^2/R
where
F = force
M = Mass
V = Velocity
R = Radius
I will assume the shark is swimming in a circle with a radius of 14 m / 2 = 7 m. The actual radius will be smaller, but I'll assume that the question is simply poorly worded. So we need to calculate the velocity of the shark. The circumference of the circle is pi*14m = 43.98229715 m. And since velocity is defined as distance over time, we have a velocity of 43.98229715 m / 80 s = 0.549778714 m/s.
Now substitute the known values into the equation for centripetal force.
F = MV^2/R
F = 681 kg * ( 0.549778714 m/s)^2 / 7m
F = 681 kg * 0.302256635 m^2/s^2 / 7m
F = 205.8367683 kg*m^2/s^2 / 7m
F = 29.40525261 kg*m/s^2
F = 29.40525261 N
Rounding to 2 significant figures gives a force of 29 Newtons.
