# Homework Help: Initial velocity when deacelerating at a constant rate

1. Sep 20, 2009

### xc630

1. The problem statement, all variables and given/known data
You are driving along a straight section of roadway (x-axis). You spot a police officer and apply the brakes, slowing down at a constant rate. Your positions (in meters) at successive time intervals of 0.435 s are tabulated below as function of time. (1) Calculate the initial speed (i.e., the speed at t=0, the time at which the brakes are first applied.). (2) Calculate your acceleration along the x-axis, ax.
x (m) 7.00 19.73 31.92 43.55
t (s) 0.000 0.435 0.870 1.305

2. Relevant equations

v= (d2-d1)/t
x= Vo(t) +1/2at^2
V= Vo + at

3. The attempt at a solution

For the initial velocity I tried using the equation x= Vo (t) +1/2 at^2
the problem says slowing down at a constant rate so I put a= 0, leaving x=vo (t) I subtracted 43.55-7 to get 36.55m over a period of the 1.305s. Dividing 36.55/1.305 i got 28 m/s but the correct one is supposed to be 29.9m/s. What did I do wrong. And for part B of the problem would I just be able to use the initial velocity from part A and the same equation I mentioned above to find a?

2. Sep 21, 2009

### Andrew Mason

Why would a = 0 if the car is slowing down? The rate of change of a may be 0 but not a.

AM