You are driving through town at 12 m/s when suddenly a ball rolls out in front of you. You apply the brakes and begin slowing down at 3.2 m/s2. When you have traveled half of the distance required to stop, what is your speed?

Question

You are driving through town at 12 m/s when suddenly a ball rolls out in front of you.  You apply the brakes and begin slowing down at 3.2 m/s2.  When you have traveled half of the distance required to stop, what is your speed?

anonymous · Answer

Are you familiar with the equations for motion in a straight line with constant acceleration ?

anonymous · Answer

Kind of? I worked it out and got 72 m/s, which doesn't make sense

anonymous · Answer

hmm, you're right, that doesn't make sense

anonymous · Answer

There might be different ways to solve this problem, but I have an idea what you need to do

anonymous · Answer

Okay

anonymous · Answer

Any idea what is the first thing you need to try to calculate ?

anonymous · Answer

I calculated the total distance and got 22.5 m

anonymous · Answer

Right, total distance is a good place to start, i haven't calculated it yet - which formula did you use ?

anonymous · Answer

Vf^2= Vi^2+ 2ax

anonymous · Answer

yes, that's what i've got, i make it 22.5
so now what's the halfway point in distance ?

anonymous · Answer

11.25- I plugged it back in and got 72 for some reason

anonymous · Answer

ok wait, you're doing it correctly
but just look carefully at the formula that you plug back into

anonymous · Answer

what does that formula calculate ?

anonymous · Answer

you already wrote it out for me a few lines ago : )

anonymous · Answer

oh my god. V^2.

anonymous · Answer

: )

anonymous · Answer

take the square root and you're in business

anonymous · Answer

Wow, I feel like an idiot. Thank you!

anonymous · Answer

no problem - and don't feel like an idiot, that's the way most stuff gets done, with some back and forth, looking for the errors !