FT=MV
F(4)=(5)(20)
F= 100/4
F= 25 N
        
             
        
        
        
This is called the Phi Phenomenon.
This is an illusion of movement created when two or more adjacent lights blink on and off in quick succession; when two adjacent stationary lights blink on and off in quick succession; we perceive a single light moving back and forth between them. It is an optical illusion of perceiving a series of still images, when viewed in rapid succession, as continuous motion. 
        
             
        
        
        
Answer:
(a) The absolute pressure at the bottom of the freshwater lake is 395.3 kPa
(b) The force exerted by the water on the window is 36101.5 N 
Explanation:
(a) 
The absolute pressure is given by the formula 

Where  is the absolute pressure
 is the absolute pressure 
 is the atmospheric pressure
 is the atmospheric pressure 
 is the density
 is the density 
 is the acceleration due to gravity (Take
 is the acceleration due to gravity (Take  )
 )
h is the height
From the question 
h = 30.0 m 
 = 1.00 × 10³ kg/m³ = 1000 kg/m³
 = 1.00 × 10³ kg/m³ = 1000 kg/m³ 
 = 101.3 kPa = 101300 Pa
 = 101.3 kPa = 101300 Pa
Using the formula
 
 
P = 101300 + (1000×9.8×30.0)
P = 101300 + 294000
P =395300 Pa 
P = 395.3 kPa
Hence, the absolute pressure at the bottom of the freshwater lake is 395.3 kPa
(b) 
For the force exerted
From 
P = F/A
Where P is the pressure 
F is the force 
and A is the area
Then, F = P × A
Here, The area will be area of the window of the underwater vehicle. 
Diameter of the circular window = 34.1 cm = 0.341 m
From Area = πD²/4 
Then, A = π×(0.341)²/4 = 0.0913269 m²
Now, 
From F = P × A 
F = 395300 × 0.0913269
F = 36101.5 N
Hence, the force exerted by the water on the window is 36101.5 N 
 
        
             
        
        
        
Explanation:
Speed or velocity (V) = 35 m/s
Kinetic energy (K. E) = 1500 Joule 
mass (m) = ? 
We know 
K.E = 1/2 * m * v² 
1500 = 1/2 * m * 35²
1500 * 2 = 1225m
m = 3000 / 1225 
m = 2.45 kg 
The mass of the object is 2.45 kg 
Hope it will help :)