The time for half the atoms in a radioactive substance to disintegrate
https://www.vocabulary.com/dictionary/half-life
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:
the changes in waves
Explanation:
the moon has its own gravitational pull thus making waves and the rising tides
It would be d all of the above
The wavelength is 2m.
Hence, Option c) 2m is the correct answer
Given that;
Frequency;
Speed; 
Wavelength; 
using the expression for the relations between wavelength, frequency and speed of wave:

Where
is wavelength, f is frequency and v is speed.
We substitute our given values into the equation

The wavelength is 2m.
Hence, Option c) 2m is the correct answer.
To learn more about wavelength, click here: brainly.com/question/1347107