# Science

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?

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asked by Melody
1. 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

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posted by drwls
2. 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.

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posted by Melody
3. 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.

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posted by drwls
4. 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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posted by Melody

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