Finding how long a collision lasts

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SUMMARY

The discussion centers on calculating the duration of a collision between a truck and a concrete wall, specifically a 2000 kg truck traveling at 23 m/s that crumples by 0.67 m upon impact. Participants determined that the average velocity during the collision is 11.5 m/s and debated the assumptions regarding constant acceleration. The conversation highlights the necessity of making justified assumptions in physics problems, particularly when estimating collision durations and forces involved.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Familiarity with concepts of average velocity and acceleration
  • Knowledge of force calculations, specifically F = ma
  • Basic principles of momentum and impulse
NEXT STEPS
  • Study the concept of impulse and its relation to collisions
  • Learn about the equations of motion under constant acceleration
  • Explore real-world applications of collision physics in automotive safety
  • Investigate the effects of varying acceleration profiles during impacts
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Physics students, automotive engineers, and safety analysts interested in understanding collision dynamics and the forces involved in vehicle impacts.

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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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