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 
average velocity is vector displacement / time
time is "almost exactly one hour"
disp = -10m
v= -10/1x60x60 = -1/360m/s
Answer:
Visible light
Explanation:
Electromagnetic spectrum is the classification of the electromagnetic waves according to their frequency/wavelength. In order from the shortest to the longest wavelength, we have
Gamma rays
X-rays
Ultraviolet
Visible light
Infrared
Microwaves
Radio waves
All these waves are invisible to human eye, except for the part referred as 'visible light'. The electromagnetic waves of this part of the spectrum are visible to human eye, and they appear as a different color depending on their wavelength. In particular, we have:
Violet: 380-450 nm
Blue: 450-495 nm
Green: 495-570 nm
Yellow: 570-590 nm
Orange: 590-620 nm
Red: 620-750 nm
<span>law of conservation of </span>energy<span> is </span><span><span>states that energy of the universe remains constant cant be created nor destroyed and conserving energy is not using as much power as you was like trying to make power bill lower while law of conservation is constant </span> </span>
Observer A is moving inside the train
so here observer A will not be able to see the change in position of train as he is standing in the same reference frame
So here as per observer A the train will remain at rest and its not moving at all
Observer B is standing on the platform so here it is a stationary reference frame which is outside the moving body
So here observer B will see the actual motion of train which is moving in forward direction away from the platform
Observer C is inside other train which is moving in opposite direction on parallel track. So as per observer C the train is coming nearer to him at faster speed then the actual speed because they are moving in opposite direction
So the distance between them will decrease at faster rate
Now as per Newton's II law
F = ma
Now if train apply the brakes the net force on it will be opposite to its motion
So we can say
- F = ma

so here acceleration negative will show that train will get slower and its distance with respect to us is now increasing with less rate
It is not affected by the gravity because the gravity will cause the weight of train and this weight is always counterbalanced by normal force on the train
So there is no effect on train motion