Seat Belt constraints in a crashing car

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Homework Help Overview

The problem involves calculating the distance required for a car's front end to collapse during a collision, given a specific deceleration and initial speed. The subject area pertains to kinematics and dynamics in physics, particularly focusing on motion under uniform acceleration.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the time it takes to decelerate from 80 km/h to 0 km/h under a specified acceleration. There are attempts to calculate distance traveled during this deceleration, with some questioning the accuracy of the initial speed conversion and the application of kinematic equations.

Discussion Status

The discussion is ongoing, with participants providing calculations and questioning assumptions made in the problem. Some guidance has been offered regarding the correct interpretation of acceleration versus velocity, and there is recognition of the need for a specific kinematic equation to approach the problem more accurately.

Contextual Notes

Participants are navigating potential misunderstandings regarding units of measurement and the nature of uniform deceleration. There is an emphasis on ensuring the correct application of physics principles without resolving the calculations themselves.

atbruick
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Homework Statement


A person who is properly constrained by an over-the-shoulder seat belt has a good chance of surviving a car collision if the deceleration does not exceed about 30 "g's" (1.0 g = 9.80 m/seconds squared).

Assuming uniform deceleration of this value, calculate the distance over which the front end of the car must be designed to collapse if a crash brings the car to rest from 80 km/h.

Homework Equations





The Attempt at a Solution

 
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How long does it take to go from a speed of 80 km/h to 0 km/h, if the speed decreases by 30x9.80 m/s every second?
What distance would one travel if moving from stand still at acceleration 30x9.80 m/s in this time?
 
For the first I got 22 m/s (instead of 80km/h) then divided that by 30*9.80m/s and got 0.075, which is the time it would take to go from 22 m/s to 0 m/s in the crash.

Then I found in that time frame a person could travel 1.65, or 1.7 meters. I hope these calculations are right; I entered 1.7 into the answer and it was wrong.
 
"30*9.80 m/s" (velocity) is not the same as "30*9.80 m/s per second" (acceleration)

There exists a specific kinematic equation that you should be using to get a better answer.
 
It appears you assumed that the car was going 22m/s the whole time during that 0.076 seconds, but the car is decelerating during that time.
 

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