# Car deceleration speed?

1. Sep 13, 2008

### nic_nic344

Hey,
Just wanted to clarify that this was the right way to calculate the deceleration when braking from a speed of 100km/h when the braking distance of the car is 45.73m.

I am just using the formula v^2 = u^2 + 2as

Which i have rearranged to find a...

a = (v^2 - u^2)/2s ---------------- s = distance

And I converted the 100km/h to m/s
so....

a = (0 - 27^2) / 2 * 45.73
a = - 7.97ms/s

Is that right???

Or should I just leave the 100 in km...

and then by deceleration is like -100ms/s?

Thanks!

2. Sep 13, 2008

### tiny-tim

Welcome to PF!

Hi nic_nic344! Welcome to PF!

Yes that's right … you get everything into the same units … except that your 27 isn't accurate enough.

3. Sep 13, 2008

### nic_nic344

Hey,
Yeh I will change the units to something like .78
Thanks for your help!!
:)

4. Sep 13, 2008

### ray b

real world the deceleration rate of a car is not steady state over the speed drop
it starts low and gets higher as speed drops