Clueless and in need of help -- I was the rear car of a 4 car collision

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Discussion Overview

The discussion revolves around the physics of a car accident involving a four-car collision, specifically focusing on the dynamics of how one vehicle (weighing 790 kg) could cause another vehicle (weighing 1523 kg) to move forward. The inquiry includes considerations of speed and impact forces, as well as the implications for the participant's situation regarding liability and insurance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • The original poster (OP) seeks to understand the speed required for a 790 kg vehicle to move a 1523 kg vehicle forward, asserting that they were traveling at less than 20 mph during the incident.
  • One participant argues that there is no minimum speed required to cause the larger mass to move, suggesting that any positive speed could suffice.
  • A rough estimate is provided, assuming an impact velocity of 15 mph and discussing the resulting motion of the combined masses after the collision, including deceleration rates and distance traveled post-impact.
  • The assumptions made in the calculations, such as inelastic collision and locked wheels, are noted as questionable, indicating uncertainty in the estimates provided.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the collision and the physics involved. While some agree on the possibility of movement at any speed, the assumptions and calculations presented are debated, and no consensus is reached on the specifics of the impact dynamics.

Contextual Notes

The discussion includes limitations related to assumptions about collision types, vehicle conditions, and the applicability of the calculations to real-world scenarios. The estimates provided are based on uncertain data and may not accurately reflect the actual circumstances of the collision.

GracieBridger
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Hi guys I'm completely useless when it comes to physics and need help with working something out regarding a car accident.
I was rear car of a 4 car collision where all cars infront had already crashed.
Now the guy infront of me is saying I shunted him which is a complete lie.
I was traveling at less than 20mph when the collision happened.

I want to know how fast a mass of 790kg would have to be traveling to cause a mass of 1523kg to move forward.

Everyone else involved in the collision has told the truth about what happened except this clown who is trying to pull a fast one!

I'll appreciate all your help!
 
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Nothing that you get from an internet forum is likely to have any weight at all either in court or in getting an insurance company to believe your side of the story so an answer really isn't going to help you convince anyone else, and YOU don't need convincing.
 
It's not going to court. My insurance do believe my story already as all other parties have said the same. We're now gathering evidence and I was interested as I think what he's saying is impossible.
I'm looking for an answer to the question, not to be spoken to like an uneducated twerp. Suppose I should say thank you for replying but then again your response wasn't particularly helpful.
 
Sorry ... my psychic powers were a bit off and I was not able to read your mind and know that you were already aware of what I posted.
 
GracieBridger said:
I want to know how fast a mass of 790kg would have to be traveling to cause a mass of 1523kg to move forward.
To answer that question directly: There is no minimum speed that could have caused a 1523 kg mass to move forward. Any positive speed would do.

One could make a crude estimate of the distance such an impact could have moved the larger vehicle. Let us assume an impact velocity of 15 mph or 22 feet per second. Further assuming an inelastic collision, the combined total mass of 2313 kg after the collision would be moving at about 7.5 feet per second.

If all wheels were locked (another questionable assumption), the deceleration rate would be about 32 feet per second per second times the coefficient of friction of locked-up tires. Estimating the coefficient of friction at 0.7, call it 0.7 times 32 = 22 feet per second per second. From 7.5 feet per second to zero at that rate would take about 1/3 of a second.

Given constant decelleration, the average speed over that 1/3 second would be half of the original 7.5 feet per second. That comes to around 15 inches covered by the impacted vehicle following the collision.

That's a rough estimate based on uncertain source data and questionable assumptions. Worth almost as much as you paid for it.
 

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