It sure sounds like someone is preparing for a court case
involving a traffic accident. Unfortunately, we can't help out too much
here, because some information is missing.
If a moving object bumps into a stationary object and they then
stick together (it sounds like this is the situation) they will
continue moving forever, unless some other force acts on them. In your
case, they came to rest (with respect to the Earth), so they probably
had friction with that object- say friction between tires and a street.
To answer your question, one then needs to know the coefficient of
friction between the tires and the street or dirt or whatever. That
depends a lot on conditions, so even if our guesses about what you're
asking about are right, we can't give an accurate answer.
Let's say, just for further guessing, that the friction was between
dry rubber and dry concrete (that would mean that the parking brakes
are on on the stationary object, or that it is being pushed sideways to
keep the tires from rolling, which would make for a very different
friction scenario). The coefficient of friction between dry rubber and
dry concrete is about 0.8. Then the combined objects decelerated at
about 8 m/s^2. It's not hard to solve that they were travelling at
about 20 m/s right after colliding. Since in a collision momentum is
conserved (not enough time for outside forces to change it) the 6400 lb
object (truck?) would have been travelling at 30 m/s before the
collision. That's 108 km/hr, or about 66 mph. If the big object weighed
8800 lb, its initial speed would have been about 27 m/s, or about 60
What was the speed limit?
(republished on 07/11/06)