Asked by Melody

I am working on a year 10 physics assignment on road science. I am trying to figure out the reaction distance of a car with:
Skid marks = 24m
Reaction time = 1 sec
Decelaration rate = 6m/sec

I have many formulas in my book and am unsure of which one to use and what the correct answer is. I keep getting different ones. Can you please help?

Answers

Answered by drwls
The reaction distance is how far the car travels before brakes are applied. That equals (reaction time)*(intial velocity)
The initial velocity (V) satisfies the equation
V = sqrt(2 a X),
where a is the deceleration rate and X is the distance travelled while decelerating. Assume that X is the skid mark length (i.e., the wheels lock as soon as the brakes are applied). Thus V = 17 m/s and the reaction distance is 17 m
Answered by Melody
Thanks for your reponse, it helped greatly. Can you please also tell me if the braking distance is 24metres and it decelerates at rate of 6m/s, does that mean that the time taken to stop after braking would be 4 seconds or do I need to consider anything else to calculate time.
Answered by drwls
The braking distance (not counting the time travelled during the reaction time) is 24 m.

Since V = 17 m/s and the average V is V/2 = 8.5 m/s while brakes are applied, the time spent decelerating is
24 m/(8.5 m/s) = 2.82 s

No, you cannot say that X/V = stopping time.

Add 1.0 s (the reaction time) to 2.82 s and you get a total time to stop of 3.82 s. This would be measured from the instant the driver decides to brake, but has not begun decelerating.
Answered by Melody
Thanks again for the help, I am now getting there with my assignment. Just one more question though. If I doubled the weight of the car, would the deceleration rate change from 6m/s to 12m/s?
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