I am thinking of an alternative timing mechanism; I am building a trap and I wanted to try something different to set it off.

I want to drop a sheep from n blocks high so that I have a delay before the trap is triggered. The timing needs to be very exact, but I don't know how the falling mechanism works: whether there is acceleration, and how fast it is hit. (I understand this would be different for a chicken.)

Can someone give me some figures of the length of time it takes to reach the terminal velocity, and how many blocks per second a mob will fall at terminal velocity?

50 bonus points for the answer for chicken speeds as an extra: This will be applied after two days; the best answer in reference to chickens will get the bounty.

What do you mean, an African or European sheep? Alex

It is an Unladen European Sheep.

Now a Post on Physics SE

The winner is confirmed as he was the sole entry but thanks for all the comments which made the answer so brilliant, as soon as the system lets me i will be giving Caleb +100 bounty