A block which its mass is $5\,kg$ is at rest as indicated. A thin bar of negligible weight has a length of $0.4\,m$ and it links it to point $A$ where there is a mechanism which allows turning about that axis. A force $P=10N$ is applied to the block as depicted. The bar has a moment of $M=3t$. Given these conditions find the speed of the block at $4\,s$.

The alternatives are as follows:

$\begin{array}{ll}

1.&5\,\frac{m}{s}\\

2.&10\,\frac{m}{s}\\

3.&15\,\frac{m}{s}\\

4.&20\,\frac{m}{s}\\

5.&25\,\frac{m}{s}\\

\end{array}$

In this problem I'm confused exactly which sort of information should I use?. Can someone help me with this matter?. I Think it has to do with $L = r\times p$ But I don't know if this is applicable to this given situation. Can someone help me here please?.