To develop this problem it is necessary to apply the concepts related to Gravitational Potential Energy.
Gravitational potential energy can be defined as

As M=m, then

Where,
m = Mass
G =Gravitational Universal Constant
R = Distance /Radius
PART A) As half its initial value is u'=2u, then



Therefore replacing we have that,

Re-arrange to find v,



Therefore the velocity when the separation has decreased to one-half its initial value is 816m/s
PART B) With a final separation distance of 2r, we have that

Therefore




Therefore the velocity when they are about to collide is 
Answer:
elements in the same column have the same number of neutrons. elements with similar mass are placed in the same column.
The legend is that he discovered gravity when an apple feel on his head. I don’t know what the true story is, but that’s what I’ve heard so maybe A??
Although, I’m pretty sure it could also be C
So... between A and C, however, I don’t want you to get it wrong so I would recommend getting another opinion
Hope this helps!
Answer:
1 x 10 -10 whisper at 1m distance.
Explanation:
- Properly fitted ear plugs an reduce noise form 15-30db. Although they are better for low frequency