Answer:
[tex]F_{net} = 31.88 N[/tex]
Explanation:
When top block is just or about to slide on the lower block then we can say that the frictional force on it will be maximum static friction
So we will have
[tex]F_{net} = ma[/tex]
[tex]F_s = ma[/tex]
[tex]\mu_s mg = ma[/tex]
[tex]a = \mu_s g[/tex]
[tex]a = (0.50)(9.81)[/tex]
[tex]a = 4.905 m/s^2[/tex]
now for the Net force on two blocks to move together
[tex]F_{net} = (m_1 + m_2) a[/tex]
[tex]F_{net} = (2.3 + 4.2)(4.905)[/tex]
[tex]F_{net} = 31.88 N[/tex]