Physics Question?
Two automobiles, moving at 65 km/h in opposite directions,
collide head-on. One automobile has a mass of 700 kg; the
other, a mass of 1500 kg. After the collision, both remain
joined together. What is the velocity of the wreck? What is
the change of the velocity of each automobile during the
collision?
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answers:
billrussell42 say: Each moving at 65 km/h ? (18.06 m/s)

momentum of first is 700x 18.06 = 12640 kgm/s (call this +)
of second is 1500x 18.06 = 27080 kgm/s in opposite direction (call this –)

conservation of momentum. Initial is –27080 + 12640 = –14440
final is the same –14440 = (700+1500)V
V = –6.57 m/s (direction of larger vehicle)


change in velocity?
for the first it is 6.57 – 18.06 (direction reverses)
for the second it is 18.06 – 6.57 (same direction, but slower)
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frank say: I don't know
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oubaas say: velocity of the wreck :
65*(1500-700)/(1500+700) = 23.64 km/h

change in velocity of the heavier vehicle ΔVh :
ΔVh = Vfin-Vin = 65.23.64= 41,36 km/h

change in velocity of the lighter vehicle ΔVℓ :
ΔVℓ = Vfin-Vin = 23.64+65 = 88,64 km/h
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Andrew Smith say: By picking the earth as a frame of reference you automatically make this problem more complex.
However conservation of momentum applies.
prior to the collision you have a momentum of 1500 * 65 - 700 * 65 = 52000 units

After the collision you have a momentum of (1500 + 700)*v
hence v = 52000/2200 ~= 23.6 km/hr in the direction of the heavier truck.
So the heavy truck has a change of velocity of 23.6 - 65 = - 31.4 ( a thirty km/hr collision)
and the light truck has a change of velocity of 23.6 - ( -65) = + 98.6 ( a 100 km/hr collision)

Now consider that a person in each vehicle has this change of velocity.

The person in the light truck has THREE times the change of velocity compared to the person in the heavy truck.

But wait. There's more. The change in the kinetic energy of the person in the light truck is then NINE times more than the person in the heavy truck. Hence the massive increase in the amount of damage to the person.
In fact an 80km/hr collision is seen as the limit of survivability. So the person in the light truck is in a non survivable collision and the one in the heavy truck is involved in a very minor bingle.

So you can see that permitting vehicles of different masses on the same roads is going to cause deaths at even a very low speed collision.
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