predict the reading of the spring balance when the wooden block is pulled on the sandpaper. Explain your answer.

* if the spring force is greater than the maximum of the static friction force,  

               Fe = m (a + μ_k g)

* If the elastic force is less than or equal to the static friction force, the result is with static friction coefficient

              Fe =μ_s m g

Explanation:

For this exercise we must apply Newton's second law to the system

X axis

         Fe -fr = m a

         Fe = m a + fr

Y axis  

        N-W = 0

        N = W = mg

the roe force is given by

        fr = μ N

        fr = μ mg

we substitute

      Fe = m a + μ m g

      Ee = m (a + μ g)

Let's analyze the solution. We have several possibilities

* if the spring force is greater than the maximum of the static friction force, the system acquires an acceleration and the result is with the kinetic friction coefficient

               Fe = m (a + μ_k g)

* If the elastic force is less than or equal to the static friction force, the result is with static friction coefficient

              Fe =μ_s m g