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:
Part a)

Part b)

Part c)

Explanation:
Part a)
As we know that force on the passenger while moving in circle is given as

now variation in force is given as

here speed is constant
Part b)
Now if the variation in force is required such that r is constant then we will have

so we have

Part c)
As we know that time period of the circular motion is given as

so here if radius is constant then variation in time period is given as

Answer:
The speed is 15 km/h or 4.16 m/s.
Explanation:
A boat travels the distance that separates Gran Canaria from Tenerife (90 km) in 6 hours. Which the speed of the boat in km / h? And in m / s?
Given that,
Distance, d = 90 km = 90000 m
Time, t = 6 hours = 21600 s
Speed = distance/time

or

So, the required speed is 15 km/h or 4.16 m/s.
<h3>Answer;</h3>
<em><u>Sand Spit or Spit </u></em>
<h3><u>Explanation;</u></h3>
- <em><u>Long shore drift is the process that occurs when a sheet of water moves on and off the beach, in other words the swash and back swash</u></em>, thus capturing and transporting sediment on the beach back out to the sea.
- <em><u>Sandbar</u></em> is normally formed when the sandspit stretches across a bay and connects the two sides. <em><u>Headland</u></em> is a high piece of land that extends out onto the sea. <em><u>Sea stacks </u></em>on the other hand results from the collapsing of the roof of the arch.