<|system|>
A particle moves with $v=$ const. along the curve $r=k(1+\cos \theta)$ (a cardioid). Find $\ddot{\mathbf{r}} \cdot \mathbf{e}_r=\mathbf{a} \cdot \mathbf{e}_r$.</s>.
<|user|>
Can you answer based on context?
<|assistant|>
-0.75