Answer
At the beginning.
Work Step by Step
The power is constant, so the work done in a time t is Pt.
This equals the change in kinetic energy. Assume the car starts from rest.
$$Pt=\frac{1}{2}mv^2$$
$$v=\sqrt{\frac{2Pt}{m}}$$
Find the acceleration.
$$a=\frac{dv}{dt}=\sqrt{\frac{2P}{m}}\frac{1}{2}t^{-1/2}$$
The acceleration is larger for small values of t.
