Here is the problem:

An Archer shoots an arrow into a piece of wood. The arrow is traveling at 120 km/h when it strikes the wood. The arrow penetrates 3.8cm into the wood before stopping. What is the average acceleration (in m/s squared) into the wood?

I am having trouble with this because there is no time given as to how long it took to go 3.8cm. Am I wrong thinking this way? Can it be solved with just the above info?
Would you please help me solve this? Thanks.

well you know that between 0cm and 3.8cm, it slows from 120km/h to a stop.

look at your suvat (http://en.wikipedia.org/wiki/Suvat) equations for an equation with does not have time as a variable, use it and you should get the answer you're looking for.

u = initial velocity = 33.33 m/per sec

v = final velocity = 0, s = distance = 3.8cm = .038 m, a = acceleration = ?

v^2 - u^2 = 2as

- 33.33^2 = 2a (.038)

a = -14620 (meter, sec)

u = initial velocity = 33.33 m/per sec

v = final velocity = 0, s = distance = 3.8cm = .038 m, a = acceleration = ?

v^2 - u^2 = 2as

- 33.33^2 = 2a (.038)

a = -14620 (meter, sec)

I don't think this was necessary, the OP just needed a poke in the right direction. I think we have a responsibility to help people learn, not just give them answers.