The chair does not move along the vertical direction: this means that the net force on the vertical direction is zero.
There are three forces acting along the vertical direction:
- The weight, acting downward: 
- The component of the force F perpendicular to the floor, acting downward: 
- The normal force N, acting upwards.
Since the net force must be zero, we have:

From which we can find the value of N:

To solve this problem it is necessary to apply the concepts related to destructive interference.
The concept refers to an overlap of two or more waves of identical or similar frequency that, when interfering, creates a new pattern of waves of lower intensity
By definition destructive interference is given by

Where,

n=integer (1,2,3,4,5,6...etc)
t = thickness
M= Index of refrqaction
For minimum thickness to satisfy this condition n will be minimum there,
n=0
Therefore

Solving to find M,



Therefore the correct answer is B. 1.38
The particles in diagram 3 move fast enough to break away from each other.
Working formula:
y = v*t + 0.5*a*t^2
where:
y = vertical distance = -1 m
v = initial velocity = 1.25m/s
a = acceleration = 9.81 m/s^2
t = time
Required: t
Solution:
y = v*t + 0.5*a*t^2
1 = (1.25)*t + 0.5*9.81*t^2
0 = -1 + 1.25t + 4.905t^2
Applying completing the square, we get
t = 0.342 s, - 0.597s
Technically, we don't have time with negative value. Thus, the answer is t = 0.342 seconds