Answer
At $t = 3.0~s$, the speed of the center of mass is $~~9.3~m/s$
Work Step by Step
We can find the speed of the car at $t = 3.0~s$:
$v = v_0+at$
$v = 0+(4.0~m/s^2)(3.0~s)$
$v = 12.0~m/s$
The speed of the truck is $~~8.0~m/s$
We can find the speed of the center of mass at $t = 3.0~s$:
$v_{com} = \frac{(1000~kg)(12.0~m/s)+(2000~kg)(8.0~m/s)}{1000~kg+2000~kg}$
$v_{com} = 9.3~m/s$
At $t = 3.0~s$, the speed of the center of mass is $~~9.3~m/s$.
