Answer:
the length of the pipe is 0.85 m or 85 cm
Explanation:
Given the data in the question;
The successive harmonics are;  700 Hz , 900 Hz , and 1100 H
Now, for a closed pipe,
length of pipe (L) = λ/4
Harmonics; 1x, 3x, 5x, 7x, 9x, 11x
1100Hz - 900Hz = 200Hz
⇒ 2x = 200Hz
x = 100Hz ( fundamental frequency )
λ = V/f = 340 /100 = 3.4 m
Now
Length L = λ / 4
L = 3.4 / 4
L = 0.85 m or 85 cm
Therefore, the length of the pipe is 0.85 m or 85 cm
 
        
             
        
        
        
Answer:
 t = 39.60 s
Explanation:
Let's take a careful look at this interesting exercise.
In the first case the two motors apply the force in the same direction
             F = m a₀          
            a₀ = F / m
with this acceleration it takes t = 28s to travel a distance, starting from rest
            x = v₀ t + ½ a t²
            x = ½ a₀ t²
            t² = 2x / a₀
            28² = 2x /a₀          (1)
in a second case the two motors apply perpendicular forces
we can analyze this situation as two independent movements, one in each direction
            
in the direction of axis a, there is a motor so its force is F/2
                
the acceleration on this axis is
           a = F/2m
           a = a₀ / 2
so if we use the distance equation
              x = v₀ t + ½ a t²
as part of rest v₀ = 0
              x = ½ (a₀ / 2) t²
              
let's clear the time
              t² = (2x / a₀)  2
we substitute the let of equation 1
              t² = 28² 2
              t = 28 √2
              t = 39.60 s