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Motion In Straight Line

Question
CBSEENPH11017212

After the engine of a moving motorboat is cut off, the motorboat retards with retardation directly proportional to v. What is the magnitude of velocity at any instant after the engine is cut off, if the velocity at the time of cut off is v0?

Solution

Given, 
Retardation of the boattau space proportional to space nu
Therefore, 
 
       r = kv; k is the proportionality constant. 
i. e. comma space straight a space equals space minus kv
Now, we know that acceleration is given by rate of change of velocity. 
Rate space of space change space of space velocity space is space known space as space acceleration. space

rightwards double arrow space dv over straight v space equals space minus space straight k space dt space

Now comma space on space integrating space both space sides comma space

integral subscript straight v subscript straight o end subscript superscript straight v dv over straight v space equals space minus space straight k integral subscript 0 superscript straight t dt space

rightwards double arrow space Ln space straight v vertical line subscript straight v subscript straight o end subscript superscript straight v space equals space minus kt vertical line subscript 0 superscript straight t space

rightwards double arrow Ln space straight v over straight v subscript straight o space equals space minus space kt space

rightwards double arrow space straight v space equals space straight v subscript straight o space straight e to the power of negative kt end exponent