I need a question Answered, Invovles Maths, Physics and cars.

[ordinaryjoe]

I made some errors in my calculations actually... I worked out the mass of the cars together as 1820kg instead of 2820kg...
And for the sake of avoiding argument i will use momentum instead of energy. If you are so determined that momentum suddenly dissipates to the Earth, please explain.


The cars decelerated at 7m/s^2 and the distance was 20m.
0=x^2-7x20 x^2=140 x=about 12m^s (speed when the 2 cars collided)
1285kg Celica travelling at y m/s has momentum 1285 y kgm/s
Together the cars have mass 2820kg and have the same momentum as the original car which hit it (1285 y kgm/s) which makes them go 12m/s initially so therefore they have momentum 2820x12 kgm/s = 33 840 kgm/s.
1285 y = 33840
y=26m/s=58.9mph

 

[wonko]

v^2 = u^2 + 2as when I last looked . You've ended up with the same result I did guessing half that decelleration.
I don't really want to get sucked into the debate but there are soooo many unknowns here - were wheels rolling or skidding, and how many were skidding, what was the friction to the road, was it level ground, what effect did crumpling have, did the impacting car brake or apply any tractive effort, maybe there was a kerb involved, etc. etc. that all we can do is make a rough 'order of magnitude' estimate and try to understand the limitations that go with it.

FWIW, I've been hit from behind (while stationary) at an estimated 30mph and with brakes on it shunted me into the car ahead. What the OP described sounded like a much higher energy (speed) event.

 

[ordinaryjoe]

v^2 = u^2 + 2as when I last looked . You've ended up with the same result I did guessing half that decelleration.
I don't really want to get sucked into the debate but there are soooo many unknowns here - were wheels rolling or skidding, and how many were skidding, what was the friction to the road, was it level ground, what effect did crumpling have, did the impacting car brake or apply any tractive effort, maybe there was a kerb involved, etc. etc. that all we can do is make a rough 'order of magnitude' estimate and try to understand the limitations that go with it.

FWIW, I've been hit from behind (while stationary) at an estimated 30mph and with brakes on it shunted me into the car ahead. What the OP described sounded like a much higher energy (speed) event.

Yes I used v^2=U^2+2as but as v = 0, v^2 = 0.

I assumed the deceleration was 7m/s. This is obviously the biggest source of error but it's an approximation... it is obvious that the OP would not get a highly accurate answer with the information he gave us. And how many time?! Crumpling does not destroy momentum (or pass it on to the Earth)

 

[JBuk]

A few years ago my mum ,dad and aunt were in a stationary car waiting to turn on an A road, they were hit up the rear at about 60mph, my aunts car was crushed to about 50% of its origional length, the car also ended up quite a distance away from where it was hit, around 100 or so yards (I forget the offical amount)

luckily everyone escaped with minor injuries

 

[crazyDAJT]

However, friction does not suddenly sky-rocket at the moment of impact, then go back to normal after.

actually to a certain extent it does, have you ever come across "Stiction" or "Static Friction".

the other car was stationary, so there would potential be a large amount of stiction to overcome to get the car to slide. the time that this Stiction is acting for would be increased whilst the two cars crumpling.

 

[wonko]

Yes I used v^2=U^2+2as but as v = 0, v^2 = 0.

I assumed the deceleration was 7m/s. This is obviously the biggest source of error but it's an approximation... it is obvious that the OP would not get a highly accurate answer with the information he gave us. And how many time?! Crumpling does not destroy momentum (or pass it on to the Earth)

Then you forgot to multiply by the 2 from the 2as term

I didn't say anything about crumpling "destroying momemtum", BTW, and I'll have to reread the other posts to try to understand what is being meant by that. I put crumpling in my list of unknowns because like other items it could have a significant effect on the decelleration by lifting braking wheels off the ground or gouging metal into the tarmac.