Answer:
A
Explanation:
The answer would be A because when feet push down on anything, the force will push down the skateboard
 
        
             
        
        
        
In order to solve the problem, it is necessary to apply the concepts related to the conservation of momentum, especially when there is an impact or the throwing of an object.
The equation that defines the linear moment is given by

where,
m=Total mass
 Mass of Object
 Mass of Object
 Velocity before throwing
 Velocity before throwing
 Final Velocity
 Final Velocity
 Velocity of Object
 Velocity of Object
Our values are:

Solving to find the final speed, after throwing the object we have

We have three objects. For each object a launch is made so the final mass (denominator) will begin to be subtracted successively. In addition, during each new launch the initial speed will be given for each object thrown again.
That way during each section the equations should be modified depending on the previous one, let's start:
A) 



B) 



C) 



Therefore the final velocity of astronaut is 3.63m/s
 
        
             
        
        
        
Answer:
block velocity   v = 0.09186 = 9.18 10⁻² m/s  and speed bollet   v₀ = 11.5 m / s
Explanation:
We will solve this problem using the concepts of the moment, let's try a system formed by the two bodies, the bullet and the block; In this system all scaffolds during the crash are internal, consequently, the moment is preserved.
Let's write the moment in two moments before the crash and after the crash, let's call the mass of the bullet (m) and the mass of the Block (M)
Before the crash
      p₀ = m v₀ + 0
After the crash
     = (m + M) v
 = (m + M) v
     p₀ = 
     m v₀ = (m + M) v                    (1)
Now let's lock after the two bodies are joined, in this case the mechanical energy is conserved, write it in two moments after the crash and when you have the maximum compression of the spring
Initial
     Em₀ = K = ½ m v2
Final
     E  = Ke = ½ k x2
= Ke = ½ k x2
    Emo = E  
    ½ m v² = ½ k x²
    v² = k/m  x²
Let's look for the spring constant (k), with Hook's law
    F = -k x
    k = -F / x
    k = - 0.75 / -0.25
    k = 3 N / m
Let's calculate the speed
   v = √(k/m)   x
   v = √ (3/8.00)   0.15
   v = 0.09186 = 9.18 10⁻² m/s
This is the spped of  the  block  plus bullet rsystem right after the crash
We substitute calculate in equation  (1)
    m v₀ = (m + M) v
   v₀ = v (m + M) / m
   v₀ = 0.09186 (0.008 + 0.992) /0.008
   v₀ = 11.5 m / s