An electrically charged element is called an "ion". A neutral element is an atom.
The magnitude of the action–reaction pair between the two boxes (A and B) will be "18.2 N".
According to the question,
Mass of box A,
Mass of box B,
Horizontal force,
From the Newton's law,
→ 
or,
→ 
Bu substituting the values, we get
→ 
→ 
→ 
We can see that between the two boxes, the action-reaction pair exist.
then,
→ 
→ 
→
(magnitude)
Thus the above solution is appropriate.
Learn more about the magnitude here:
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U need to set up n solve the general eqn for simple harmonic motion:
x" = -(k/m)x
solution is x(t) = (x0)*cos(wt) + (v0/w)*sin(wt)
where w=sqrt(k/m), x0 is x-position at t=0 and v0 is vel at t=0
u already calculated f in Q.2 and w = 2*pi*f
x0 is 0 as it starts at eqm
v0 is given at 5.1
so u have x(t)
vel is given by x'(t) = (x0)*(-w)*sin(wt) + (v0/w)*w*cos(wt)
substitute t=0.32, x0=0, v0=5.1 n w in the above, u can solve for v at t=0.32.