Physics - proving the stopping distance of a car

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Homework Help Overview

The discussion revolves around calculating the stopping distance of a car given its initial speed, reaction time, and acceleration. The problem is situated within the context of kinematics, specifically dealing with motion under uniform acceleration.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the application of kinematic equations to determine stopping distance, questioning the assumptions made about the timing of the driver's reaction and the application of brakes. There is a focus on clarifying the initial conditions and the sequence of events leading to the car's stopping.

Discussion Status

Some participants have provided guidance on the interpretation of the problem, suggesting the need to consider the distance traveled during the reaction time separately from the distance during braking. Multiple interpretations of the problem are being explored, particularly regarding the initial and final velocities.

Contextual Notes

There is a noted assumption that the driver takes time to react before applying the brakes, which affects the overall stopping distance calculation. Participants are also discussing the need to account for both the distance covered during the reaction time and the distance covered while decelerating.

totomyl
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Homework Statement


an accident occurs up ahead on the highway. a driver traveling at 120km/h [e] reacts in 0.20s and applies the brakes causing an acceleration of 8.0m/s2 [w]. show that the stopping distance is 76 m.
what am i doing wrong? i changed the acceleration to match the directions, so i made it negative. but i am not getting the right answer.

Homework Equations


d = vi * t + 0.5(a * t^2)

The Attempt at a Solution


i attempted this by using:

d = (120km/h / 3.6[e])(0.20s) + 0.5(-8.0m/s[e] * 0.20s^2)

d = 6.5m?
 
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totomyl said:

Homework Statement


an accident occurs up ahead on the highway. a driver traveling at 120km/h [e] reacts in 0.20s and applies the brakes causing an acceleration of 8.0m/s2 [w]. show that the stopping distance is 76 m.
what am i doing wrong? i changed the acceleration to match the directions, so i made it negative. but i am not getting the right answer.

Homework Equations


d = vi * t + 0.5(a * t^2)

The Attempt at a Solution


i attempted this by using:

d = (120km/h / 3.6[e])(0.20s) + 0.5(-8.0m/s[e] * 0.20s^2)

d = 6.5m?
The problem is you have assumed that the car goes from 120 kph to 0 kph in 0.2 s, which is not what the problem states. The driver takes 0.2 s to press the brake pedal after he sees the accident ahead of him.

You should pick another SUVAT equation which relates distance, acceleration,and initial and final velocity.
 
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SteamKing said:
The problem is you have assumed that the car goes from 120 kph to 0 kph in 0.2 s, which is not what the problem states. The driver takes 0.2 s to press the brake pedal after he sees the accident ahead of him.

You should pick another SUVAT equation which relates distance, acceleration,and initial and final velocity.
So, I just have another question, in this question the final velocity would be at a stop, so 0 m/s correct? and also would i have to find the distance traveled before pressing on the brakes and add it to the distance it took while slowing down?
 
totomyl said:
So, I just have another question, in this question the final velocity would be at a stop, so 0 m/s correct? and also would i have to find the distance traveled before pressing on the brakes and add it to the distance it took while slowing down?
Yes and yes.
 
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SteamKing said:
Yes and yes.
thank you, your answers have been very helpful and i am very grateful!
 

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