# Homework Help: How close can you get to a stop sign before you start braking?

1. Sep 20, 2007

### bbface_assassin

1. The problem statement, all variables and given/known data
1) A typical car's brakes can create a maximum acceleration of less than 5 m/s^2 . How
close can you get to a stop sign before you start braking?

2. Relevant equations
no idea at all since there isn't even enough information given to solve this.

3. The attempt at a solution

I don't think it's even doable, since there's no initial velocity. Or am I wrong?

2. Sep 20, 2007

### tony873004

I'm stumped too. You need an initial velocity and a more exact acceleration than > 5m/s^2. You could do the problem with 5 m/s^2 and phrase your answer as "it needs at least x meters". But you still need an initial velocity. As your initial velocity approaches 0, your stopping distance also approaches 0. What if you were already at a stop? Then your stopping distance would be 0, regardless of how good your brakes were.

Is this a trick question and we just don't get the trick? Are you sure you worded it right?

3. Sep 20, 2007

### bbface_assassin

I am sure I worded it right.

Actually, I just copied it straight out of the textbook.

I tried to put the Vi as 0, but that doesn't make sense. And what am I suppose to put the acceleration as? I can't really put it as <5m/s^2. :S

4. Sep 20, 2007

### Irid

You must find a distance, measured in units of length (m). You are given acceleration, which has units of (m/s^2). Thus, you MUST have some other physical quantity, which involves units of time (s), or you won't be able to transform acceleration in such a way so as to get distance. I'd suggest you assume a typical acceleration as allowed in the city streets, such as 50 km/h and then the problem becomes fairly easy.

5. Sep 20, 2007

### J77

Yeah -- it could be a question to sort of those with a bit of common sense from those who read/do questions like robots.

If I came across a question in a test, the first thing I would do is write something like, "As no velocity is given, I will assume the car is travelling along a flat road at 50 km/hr."

If I was setting school tests, I'd always include a question which needed some common sense assumptions

6. Sep 20, 2007

### Capuchin

Isn't it just asking you to find x in terms of vi?

No need to be sticking assumed numbers in.

7. Sep 20, 2007

### tony873004

That makes sense. Perhaps the answer is expected to be a formula. You'd still need to preface your answer with "no further than" and then use 5m/s/s as your acceleration.

8. Sep 20, 2007

okay given a certain speed you can calculate the stopping distance
x= [m]
v= [m/s]
a= 5 [m/s^2]
t= braking time
for a certain speed you calculate braking time:
t=v/a
x=(1/2)*a*t^2
as stated above t=v/a so -> t^2=(v/a)^2.
x=(1/2)*5*(v/5)^2
x= 2.5*(v^2)/(5^2)
x=0.1*v^2
praise me!