Calculate velocity at a certain time in an acceleration.

[Viktor Haugland]

A car accelerates from 36 km/h to 80 km/h o range of 500m.
What's the speed at 250m?
Answer with two value figures and the unit "km/h".


Am I using the wrong formulas or something?
What am I missing?

36 km/h = 10 m/s
80 km/h = 22,22 m/s

avg. speed = 0,5(22,22 m/s + 10 m/s) = 16,11 m/s
time = 500 m / 16,11 m/s = 31,03 s
acceleration = (22,22 m/s - 10 m/s) / 31.03 = 0,39 m/^sSo I'm thinking like this (but it's wrong for some reason):

10 m/s + 0.5 * 0,39 m/^s * (31,03 s / 2) = 13,05 m/s

13,05 * 3600 / 1000 = 46.98 km/h

Answer: 47 km/h

Any thoughts?

 

[PeroK]

Two thoughts:

1) What makes you think the car is at the halfway point at half the time?

2) What you have is a lot of numbers and it's very difficult to see what you're doing or to spot a mistake. It should be a lot easier if you work with $suvat$ symbols.