Think of it like this, gravity has to pull harder on the heavier object to make them fall at the same rate , but doesn't have to pull as hard for the lighter object , thus is why sometimes heavier objects fall faster then lighter ones
Momentum is conserved throughout this scenario.
Before the man does anything, the total momentum of him and his book is zero. So we know that it'll be zero after he throws the book.
Momentum = (mass) x (velocity)
The man gives the book (1.2 kg)x(10 m/s north) = 12 kg-m/s north
of momentum.
Since the total momentum must be zero, the man himself picks up 120 kg-m/s of momentum south.
(his mass)x(his v) = 120 kg-m/s south = (770 kg-m/s^2/9.8 m/s^2)x(V).
His velocity southward = (120 x 9.8) / (770) m/s .
He needs to reach the shore 10m away.
Time = distance/speed
= (10 x 770) / (120 x 9.8) seconds
= 6.55 seconds
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