Kinematics problem: Determine the velocity of this car during braking

[dbag123]

Homework Statement

Car starts to brake at t=0s from 30m/s Determine the velocity of the car at 4s and the integration constant

Relevant Equations

a(t)=-0.6t^2+2t-8 (m/s^2)

Step 1. Integrate the function --> it comes out to -1.8t³+t²-8t+c
Step 2. Figure out the integration constant --> I know that the velocity of the car is 30m/s at t=0s so making 30m/s = -1.8(0s)³+(0s)²-8(0s)+C it would seem that C is 30m/s
Step 3. Calculate velocity at 4s --> V(4s)= -1.8(4s)³+(4s)²-8(4s)+30m/s and that equals to -101.2m/s
Am i in the right?

 

[Orodruin]

You should have a second look at your first step. What do you get if you take the derivative of your result?

 

[dbag123]

allrighty it is wrong it should be -0.2t³...
integration constant does not change but the v(4s) now becomes 1.2m/s
it should be right