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

#### GracieBridger

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 travelling at less than 20mph when the collision happened.

I want to know how fast a mass of 790kg would have to be travelling 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!

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#### phinds

Gold Member
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.

#### GracieBridger

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.

#### phinds

Gold Member
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.

#### jbriggs444

Homework Helper
I want to know how fast a mass of 790kg would have to be travelling 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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