derivative with respect to time ?? 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 !!!!!