Problem $2$: A beam is constructed by nailing together planks in the configuration shown below. If the maximum shear force that the nails can transfer is $1$ kN $,$ and the nails are all spaced $15$ cm apart along the length of the beam, what is the maximum vertical shear force that the beam can hold? Assume $\backslash (\backslash$mathrm$\{$l$\}=30\backslash$mathrm$\{$~cm$\},\backslash$mathrm$\{$w$\}=15\backslash$mathrm$\{$~cm$\}\backslash ),$ and the thickness of all the planks is $2$ cm $.$