The problem is: You apply your brakes to stop for a traffic light, and you come to a stop in a distance of 35 meters. How much time did it take you to stop?
from previous problems, i know:
INITIAL VELOCITY: 15 m/s
FINAL VELOCITY: 0 m/s
ACCELERATION: -3 m/s^2 (because you're slowing down)
DISPLACEMENT: 35 m
so we're solving for time.
i used the displacement= (initial velocity x delta t) + 1/2 (a)(delta t)^2 and got 3.7 seconds (used quadratic formula).
35= 15 (delta t) + 1/2 (-3) (delta t)^2
then put that into the quadratic formula.
[-15 +/- root 225-(4)(-1.5)(-35)]/-3
then (-15 +/- 3.87) / -3.
however, according to the answer sheet, the answer should be 4.7 seconds, not 3.7.. what did i do wrong?
See:
http://www.jiskha.com/display.cgi?id=1253754718