The recoil velocity of the astronaut is -0.070 m/s
Explanation:
We can solve this problem by using the principle of conservation of momentum: in fact, in absence of external forces, the total momentum of the astronaut-wrench system must be conserved.
At the beginning, their total momentum is zero:
(1)
Later, after the astronaut throws the wrench, the total momentum is
(2)
where
m = 0.725 kg is the mass of the wrench
v = 13.8 m/s is the velocity of the wrench
M = 143 kg is the mass of the astronaut
V is the recoil velocity of the astronaut
Since momentum is conserved, (1) = (2), and so we can find V:

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