Final answer:
The man must exert a force of 1.2 Newtons.
Step-by-step explanation:
The force the man must exert to prevent the block from falling can be calculated using the equation:
Ffriction = μs * N
where Ffriction is the force of friction, μs is the coefficient of static friction, and N is the normal force.
In this case, the normal force is equal to the weight of the block, which is 3.0 N.
Substituting the given values, we have:
Ffriction = 0.40 * 3.0 N
Ffriction = 1.2 N
Therefore, the man must exert a force of 1.2 Newtons to prevent the block from falling.