How Does Angus Macbangus's Head-butt Physics Work?

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Angus Macbangus accelerates to a head-butting speed of 8.5 m/s, with an effective head mass of 24 kg. The impulse produced by Angus can be calculated using momentum and impulse equations. To reach his speed in 0.5 seconds, the power output of his muscles is also determined. Upon impact, if Angus's head comes to rest in 0.15 seconds, the force experienced by the opponent's face is equal in magnitude but opposite in direction to the force exerted by Angus. The calculations and understanding of the physics involved appear to be correct.
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1. Problem statement and all known variables
The world champion head butter Angus Macbangus can accelerate himself from rest to head-butting speed of 8.5m/s. Since the upper body is involved in a properly executed head-butt, the effective mass of the head is 24kg. (Label the direction the head moved as +i. Assume linear motion at the moment of impact.)

a) what impulse does Angus produce?
b) if Angus reaches butting speed in 0.5s, what power must his muscles produce?
c) if Angus head-butts an opponent in the face and his head comes to rest in a time of 0.15s, what is the magnitude and the direction of the force that the opponents face experience?

2. Related equations
Momentum/impulse equations
Kinetic energy 1/2mv^2

3. Attempt
Just checking to see if my work is correct.
For part c, the force that the opponent experiences is the equal to the force I found but in the opposite direction?

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All looks about right.
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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