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**Resistance -- fluid resistance to ball motion**

A ball of mass m = 3 kg is falling from an initial height y = 3 m in an area where the acceleration due to gravity can be approximated as g = 10 m/s2. As it falls the ball is subjected to a fluid resistance of magnitude FR = kv where k = 6 kg/s. How many seconds will it take for the acceleration of the ball to decrease to a value a = 9 m/s2? Type the numerical value only, not the unit. Round off your answer to 3 decimal place.

## The Attempt at a Solution

I did FR=kv

FR=mg

3(10)=30

30=6v

v=5m/s

I don't know if this step is right...

v=v0+at

v=at

5=9t

5/9=t

PART B: I have no clue on this one.

Consider the problem of an object falling from rest in a fluid where the resistance is FR = kv. How many time constants will it take for the object to reach a velocity that represents 72% of its terminal velocity? The time constant is the ratio m/k where m is the mass of the falling object and k is the coefficient of the resistance force. Round off your answer to 1 decimal place.

Any help would be appreciated! Thanks