Shadowpaint you asked about an asteroid called Apophis and why it would do so much damage to the earth, this is this why. There are two types of energy:

1 poteintial energy
2 kinetic energy

Potiential energy is stored energy, like a boulder at the top of a hill, then if the boulder is pushed off the hill then energy that is stored energy is then changed into kinetic energy, it is the kinetic that does the damage.

The asteroid has a mass of 5 x 10^10 kilograms, it's velocity = 12590 meters/second (approx).

Energy kinetic = 1/2 x mass x velocity^2 so

= 1/2 x (5.0 x 10^10 kilograms) x (12590 m/s)^2
= 3.9627 x 10^18 joules

To convert this to 1 atomic bomb = 500,000 tons of tnt

3.9672 x 10^18 x 2.38095 x 10^-10(conversion factor for tons of tnt)
= 943,499,056.5 tons of tnt

= 1886.998 atomic bombs

Does this anwser you question?