The important aspect of this is that the asteroid would probably hit with a speed of thousands of miles per hour.
You could calculate the mass of the asteroid if you chose. Use for example the density of granite and a volume of (4/3)pi (50)^3. Then calculate the momentum using a speed for example of several thousand miles per hour.
However if you just want a word description of what would happen look up on Google or elsewhere the record of previous encounters, for example one in Siberia that devastated a large area of forest that was luckily not a city.
In addition to the information that Damon has provided, you may find this information helpful and interesting:
Typical asteroid impact speeds, relative to earth, are usually in a range of tens of thousands of mph, roughly equlaivalent to a few times the earth's orbital speed. Because impact energy is proportional to the square of the relative speed, it is important to use an accurate speed when making estimates of asteroid impact effects.
The subjects of asteroid impact modeling and search for asteroids likely to hit the Earth in the next century are active areas of research.