Answer:
58.27 N
Explanation:
the data we have is:
mass: 
coefficient of friction: 
and we also know the acceleration of gravity is 
We need to do an analysis of horizontal and vertical forces acting on the object:
-------
Vertically the forces acting on the object:
- Normal force
(acting up from the object)
- weight:
(acting down from)
so the sum of forces in the vertical axis "y" are:

from Newton's second Law we know that
, so:

and since the object is not accelerating in the vertical direction (the movement is only horizontal)
, and:

-----------
now let's analyze the horizontal forces
- frictional force:
and since
--> 
- force to move the object:

and the two forces just mentioned must be opposite, thus the sum of forces in the "x" axis is:

and we are told that the crate moves at a steady speed, thus there is no acceleration: 
and we get:

substituting known values:

By tightening a string you are actually putting more stress on the string you are giving it a new frequency that isn't natural.
Hope this helps
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Answer: reaction force = -558N
Explanation:
w = f = 558N
since action force and reaction force are equal in magnitude and opposite in direction,
reaction force = -(f)
reaction force = -558N
if that helps.
There are correlation and causation between the force of the finger and the movement of the books