Finding how long a collision lasts

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Homework Help Overview

The problem involves a truck colliding with a concrete wall during a crash test, focusing on determining the duration of the collision and the forces involved. The subject area includes concepts of kinematics and dynamics.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the average velocity during the collision and question how far the rear bumper travels. There is also a consideration of the assumptions made regarding constant acceleration during the collision.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some have provided guidance on the assumptions needed to approach the solution, while others have raised concerns about the validity of those assumptions.

Contextual Notes

There is a mention of the need to assume a particular time profile for acceleration, which some participants question as potentially unjustified. The original poster's approach to calculating average velocity and distance traveled is also noted.

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Homework Statement


In a crash test, a truck with mass 2000 kg traveling at 23 m/s smashes head-on into a concrete wall without rebounding. The front end crumples so much that the truck is 0.67 m shorter than before.

Homework Equations


About how long does the collision last? (That is, how long is the interval between first contact with the wall and coming to a stop?)
Δt = (c) What is the magnitude of the average force exerted by the wall on the truck during the collision?
Fwall, avg = N(d) It is interesting to compare this force to the weight of the truck. Calculate the ratio of the force of the wall to the gravitational force mg on the truck. This large ratio shows why a collision is so damaging.
[Fwall, avg]/mg =


The Attempt at a Solution


Well I figured out that the Average velocity during the collision was 11.5 m/s. But after this I am stuck.
 
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Ok, how far does the rear bumper travel during the collision?
 
Wouldn't it have only traveled .67 m?
 
Yes, so now can you solve for Δt?
 
Oh, duh. Thanks.
 
paisiello2 said:
Yes, so now can you solve for Δt?
Not so fast, literally.
The speed will not be constant. The acceleration will not be constant either, but you will have to assume a particular time profile for the acceleration in order to solve the problem. The questioner probably expects you to assume constant acceleration.
 
I believe the OP did this already.
 
Yep.
 
O
Ghost4512 said:
Yep.
Ok, but my point is that to do this you had to make an assumption that was not only unjustified but in practice likely to be false.
 

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