"The Drone Danger"

"...traveling at
250 miles per hour (in the U.S., that's the maximum
speed when flying below 10,000 feet) hitting a 25-pound
UAV would create a huge amount of force — around
40,000 pounds of it. The bigger the drone, the greater
the potential damage from the impact forces, shrapnel,
etc. Most hobby drones are a lot lighter than 25 pounds
and don't fly very high, posing little danger. But bigger,
heavier machines are out there, and we'll be seeing more..."

http://www.askthepilot.com/the-drone-danger/