Hey,(adsbygoogle = window.adsbygoogle || []).push({});

I have been set this question and I thought I'd answered it perfectly but it turns out I haven't:

A person driving her car at 43 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 can accelerate from 43 km/h to 70 km/h in 8.1 s. Ignore the length of her car and her reaction time. If she hits the gas, how far will she travel before the light turns red?

I decided to use s = ut + ½at² as it seemd the most appropriate, here's what I got:

s = distance travelled = unknown

u = initial speed = 43km/h = 11.944m/s

t = time = 2.0s

a = acceleration = (70km/h - 43km/h) / 8.1s = 3.333m/s/s

s = (11.944 x 2) + (½ x 3.333 x 2.0²)

s = 23.888 + 6.666

s = 30.555m

So I got the distance travelled by the car in the 2 seconds before the light turned red if the driver sped up to be 30.555m but my teacher tells me this is wrong...anyone know why?

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Mechanics-related car question

**Physics Forums | Science Articles, Homework Help, Discussion**