(a) The system of interest if the acceleration of the child in the wagon is to be calculated are the wagon and the children outside the wagon.
(b) The acceleration of the child-wagon system is 0.33 m/s².
(c) Acceleration of the child-wagon system is zero when the frictional force is 21 N.
<h3>
Net force on the third child</h3>
Apply Newton's second law of motion;
∑F = ma
where;
- ∑F is net force
- m is mass of the third child
- a is acceleration of the third child
∑F = 96 N - 75 N - 12 N = 9 N
Thus, the system of interest if the acceleration of the child in the wagon is to be calculated are;
- the wagon 
- the children outside the wagon
<h3>Free body diagram</h3>
            →                 →              Ф                         ←
          1st child      friction       wagon                2nd child
<h3>Acceleration of the  child and wagon system</h3>
a = ∑F/m
a = 9 N / 27 kg
a = 0.33 m/s²
<h3>When the frictional force is 21 N</h3>
∑F = 96 N - 75 N - 21 N = 0 N
a = ∑F/m
a = 0/27 kg
a = 0 m/s²
Learn more about net force here: brainly.com/question/14361879
#SPJ1
 
        
             
        
        
        
As it is given that the air bag deploy in time

total distance moved by the front face of the bag

Now we will use kinematics to find the acceleration




now as we know that

so we have

so the acceleration is 400g for the front surface of balloon
 
        
             
        
        
        
friction is the resistance that one surface or object encounters when moving over another. Due to gravity pulling everything down things need to friction in order to move 
i hope this helps :/
 
        
             
        
        
        
Answer:
<h2>A. a spring & B. a well dried into an aquifer.</h2>