Answer: 45 joules of energy
Explanation:
The potential energy of the block is A) 490 J
Explanation:
The potential energy of an object is the energy possessed by the object due to its position in the gravitational field.
It is calculated as follows:

where
m is the mass of the object
g is the acceleration due to gravity
h is the height of the object above the ground
For the block in this problem, we have:
m = 10 kg

h = 5 m
Therefore, its potential energy is:

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Acceleration = (final velocity - initial velocity) / time
= (35-65)/10
= -3 m/s2
The electrostatic force between the two ions is 
Explanation:
The electrostatic force between two charged particle is given by Coulomb's law:

where
is the Coulomb's constant
are the two charges
r is the separation between the two charges
In this problem, the ion of sodium has a charge of

while the ion of chlorine has a charge of

And the distance between the two ions is

Substituting, we find the electrostatic force between the two ions:

where the negative sign simply means that the force is attractive, since the two ions have opposite charge.
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