- #1

- 2

- 0

It's been a looong time since I've had to use/apply basic physics but i'm hoping i've come to the right place for help.

I am trying to help my brother-in-law with a speeding charge.

__First question__The police allege he reached 100km/h within 38m of a corner.

The first question relates to the initial velocity a vehicle would have needed to be travelling in order to reach 100km/h (27.8 m/s) in 38m.

I calculate that as this vehicle can accelerate from 0-100 km/h in 8.5, it would need 118.2m to get to 27.8 m/s from a standing start.

But I am unsure how to calculate what the initial velocity would need to be for this vehicle to reach 27.8 m/s in 40m.

__Seconds question__The police also allege he was still travelling at 27.8 m/s 47m from a speed hump. Assuming 1 sec reaction time, I calculate that he would have been travelling at:

Formula used to calculate braking distance:

Vf2 = V02 - 2ad

where Vf is the final velocity, V0 is the initial velocity, a is the rate of deceleration and d is the distance travelled during deceleration. Since Vf will be zero when the car has stopped:

d = v02 / 2a

(I've assumed a = 10 m/s - is this realistic?)

d = 772.8 / 20 = 38.6m

Stopping distance incl. 1 sec reaction time = 38.6m + 27.8m = 66.4m

So, velocity after 47m braking:

Vf = Sq root (V02 - 2ad)

= Sq root (772.8 -2 x 10 x 19.2)

= 19.7 m/s

= 71 km/h

(where d = 47 metres minus the reaction distance of 27.8 metres = 19.2 metres)

At 71 km/h he would have done some serious damage to his car.

Thanks for your help - I apologise for my slopping logic in advance