derivative with respect to time ??

[ddoerr]

ok i got a question that reads the accel of a marble is a certain liquid is given by the following equation: a=§ v^2
where §=-4.8 and v> 0m/s the initial volociy is 1.72 m/s. how long will it take the marble's volocity to be reduced to half of it's initial which is 0.86 m/s?

i think i have to take a derivative with repect to time to get the volocity equation and kinamatics doesn't work i can't just find the accel with the equation cuz the accel has an exponetial relation to volocity...i need hlep. i need to know how to take a derivative with repect to time, i can do derivative fine but with repect to time i;m stumped. HELP !!!!! :confused:

[Nenad]

do you know the answer?

[ddoerr]

nm i got it dv/dt then integrate

[marlon]

Try this a=(dv/dt) and put this in a=-4,8v^2. Then write all v's to one side and the t's to the other to get : (dv/(v^2))=-4,8dt and start integrating.

regards
marlon