A person driving her car at 54 km/h approaches an intersection just as the traffic light turns yellow.She knows that the yellow light lasts only 2.0 s before turning to red, and she is 30 m away from the near side of the intersection. The intersection is 15 m wide. Her car's maximum deceleration is -5.0 m/s2, whereas it can accelerate from 54 km/h to 70 km/h in 4.8 s. Ignore the length of her car and her reaction time.
If she hits the brakes, how far (d) will she travel before stopping?
Vin = 54/3,6 = 15.00 m/sec
d = (Vo^2-Vin^2)/2a = (0-15^2)/(2*-5) = -225/-10 = 22.5 m ....safely well before the stopping line
If she hits the gas instead, how far (d1) will she travel before the light turns red?
Vfin = 15 +(70-54)*2/(4.8*3.6) = 16.85 m/sec
d1 = (15+16.85)/2*2 = 31.85 m ....just two meters after the stop line, i.e. inside the cross area
she'd better braking