Is My Calculation for Car Deceleration Speed Correct?

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Discussion Overview

The discussion revolves around the calculation of car deceleration when braking from a speed of 100 km/h over a distance of 45.73 m. Participants explore the application of the kinematic equation v² = u² + 2as to determine the deceleration and discuss unit conversions.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents a calculation for deceleration using the formula a = (v² - u²)/2s, converting 100 km/h to m/s.
  • Another participant agrees with the approach but suggests that the conversion of 100 km/h to m/s may not have been accurate enough.
  • A later reply introduces the idea that in real-world scenarios, the deceleration rate is not constant and increases as speed decreases.

Areas of Agreement / Disagreement

Participants generally agree on the method of calculation but express differing views on the accuracy of unit conversion and the nature of deceleration in real-world conditions.

Contextual Notes

There are unresolved aspects regarding the accuracy of the speed conversion and the assumption of constant deceleration in the calculation.

nic_nic344
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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!
 
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Welcome to PF!

nic_nic344 said:
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...

Hi nic_nic344! Welcome to PF! :smile:

Yes that's right … you get everything into the same units … except that your 27 isn't accurate enough. :wink:
 
Hey,
Yeh I will change the units to something like .78
Thanks for your help!
:)
 
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
 

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