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 
2/5 = .4
.4*100= 40%
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Answer: Less than 4 ohms
Explanation:
We have three resistors with the following resistance:



Now, when the resistors are connected in parallel, the total resistance
is calculated as follows:

Isolating
:

Rewriting with th known values:

Finally:

Hence, the correct option is less than 4 ohms.
Answer:
Kinetic energy
Explanation:
Kinetic energy is a function of velocity. Since the rider is moving at a certain speed, he's demonstrating kinetic energy. It can't be potential energy because potential energy encompass mgh
Answer:
Charge Z can be placed at <em>x</em> = -2.7 m or at <em>x</em> = 0.27 m.
Explanation:
The Coulomb force between two charges,
and
, separated by a distance,
, is given

<em>k</em> is a constant.
For the charge Z to be at equilibrium, the force exerted on it by charge X must be equal and opposite to the force exerted on it by charge Y.
It is to be placed along the <em>x</em>-axis. Hence, it is on the same line as charges X and Y.
Let the charge on Z be <em>Q</em>. It is positive.
Let the distance from charge X be <em>x m.</em> Then the distance from charge Y will be (0.60 - <em>x</em>) m.
Force due to charge X

Force due to charge Y

Since both forces are equal and opposite,







Applying the quadratic formula,

or 
Charge Z can be placed at <em>x</em> = -2.7 m or at <em>x</em> = 0.27 m