# Kinematics problem help

1. Nov 8, 2007

### nblu

Q: The average alcohol-free driver requires about 0.8 s to apply the brakes
after seeing an emergency. Calculate the distance travelled after seeing the emergency
before applying the brakes.

with the question, the given speed is 17 m/s and t = 0.8.
at first, i used d =vt equation to find the distance, however,
when i read through the question again, i had a feeling that it's not right.
my first answer was d=14m, but it just doesnt look correct.

i was thinking of finding the acceleration then use v$$^{2}_{f}$$=v$$^{2}_{i}$$ +2a$$\Delta$$d
to solve for d, OR, use this equation, $$\Delta$$d=1/2(v$$_{1}$$+v$$_{2}$$)$$\Delta$$t, without
even having to find the acceleration.

one part of my knowledge tells me that i don't need acceleration to do this question
because this is about "before" applying the break.

its confusing me..

any help would be greatly appreciated.

thank you

Last edited: Nov 8, 2007
2. Nov 8, 2007

### Dick

You "know" what you did is right. Resist the temptation to complicate it. I'm sure you'll get a chance to use that other stuff later.

3. Nov 8, 2007

### Siracuse

If Dick's reply isn't enough, your logic is right to me, simple enough. I don't see how it doesn't "look correct".

4. Nov 8, 2007

### nblu

Thank you, I really appreciate it! :)