A decagram is 1000 times bigger than a centigram
·The acceleration of gravity is proportional to
1 / (the square of the distance from the center) .
When we're on the surface, we're 1 radius from the center of the Earth,
and the acceleration of gravity is 9.8 m/s² .
The boy's weight = (mass) · (gravity) = (50kg) · (9.8 m/s²)
= 490 newtons .
At the distance of 5 radii from the center (4 radii altitude from the surface),
the acceleration of gravity is
(9.8 m/s²) · (1/5)² = 0.39 m/s² .
The boy's weight is (mass) · (gravity) = (50kg) · (0.39 m/s²)
= 19.6 newtons .
Just as we expected, his weight at that distance is
(19.6 / 490) = 0.04 = 1/25 = 1/5² of his weight on the surface.
In a direct current the electric charge flows in one direction.
in an alternating the electric charge changes in its direction periodically.
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Formula which describes v in terms of t is 
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Explanation:
A train travels the distance between the points A and B, which is 600 km
Distance between A and B = 600 km
Speed of train = v
Time taken by train = t
We know that
Distance traveled = Speed of travel x Time taken
600 = v x t

Formula which describes v in terms of t is 
= (3,760 joule/sec) / (4,000 joule/sec)
= 3,760 / 4,000 = 0.94 = 94%