Which formula to use

1. Dec 7, 2014

laneschmidt

This is a classic case of using the wrong formula for the question. My problem is that, using the given information, both formulas could've been used (to the best of my knowledge)

1. The problem statement, all variables and given/known data
A car travelling at 22.4 m/s comes to a stop in 2.55 s. Determine the stopping distance of the car, assuming that acceleration is uniform.

2. Relevant equations
Let d=displacement
d=Vi*t+0.5at2
d=(Vi+Vf)/2t
a=∆v/t

3. The attempt at a solution
1. I determined that the rate of acceleration was 8.78 m/s^2→ 22.4 m/s/2.55 s = 8.78 m/s2
2. I had all the variables needed so I used the above equation:
d=(22.4 m/s)(2.55 s) + 0.5(8.78 m/s2)(2.55)2

3. After some confusion I decided to try another equation, the latter equation above:
d=(22.4 m/s + 0 m/s)/2(2.55 s)

This confuses me. Clearly I do not know when to use each equation, even when both could be used with respect to the variables given. Some insight on this would be appreciated! Thank you.

2. Dec 7, 2014

Staff: Mentor

Your first method is correct if you use the correct (negative) sign for the acceleration.

Chet

3. Dec 7, 2014

laneschmidt

Thanks Chet! I overlooked the fact that the car was decelerating.