A 1000kg boat is traveling at 90 km/h when its engine is shut off. The magnitude of the frictional force fk between boat and water is proportional to the speed V of the boat: fk = 70V where V is in meters per second and fk is in newtons. Find the time required for the boat to slow to 45 km/h.(adsbygoogle = window.adsbygoogle || []).push({});

Attempted to solve:

Take upwards as positive for y direction, x + from left to right

From newton's law 2, we have:

-fk = ma

a = -fk/m = -70Vi / m

(Vi = V initial)

V = Vi + at

---------> t = (V - Vi) /a

t = - m(V - Vi) / 70Vi (h)

with m = 1000kg, V = 45km/h, Vi = 90 km/h

However, my answer is not correct from the book. Can someone point out where I did wrong ? Thanks

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# Homework Help: Force and motion in Newton's laws

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