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April 19, 2014

Homework Help: Physics

Posted by Confused on Tuesday, October 7, 2008 at 10:46pm.

A car is going 20m/s when they see a traffic light 200m ahead turn red. The traffic light is known to stay red for 15s. The driver wants to reach the light as it turns green. It takes them 1s to press the breaks and the car slows down at a constant rate. What is the speed that she reaches the light the instant it turns green?

I think I am just missing something, but I have no idea how to find the speed..there are to many unknowns. Basically I know that vf^2=vi^2 +2a(xf-xi) but I don;t know accleration and I can't seem to figure out how to find it in order to solve, and somehow when I tried I got an answer that was larger then the initial velocity, which is impossible since its slowing down...help?

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