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Throughout schooling we are given problems in which we are to solve for velocity, force, mass, etc. I have a real life situation in which I would love to solve these exact things. Please consider: Car A is a minivan stopped at a driveway, waiting to turn southbound across traffic. Car B is a large vehicle in the right lane of the northbound traffic, velocity unknown, but assumed as going plus or minus the speed limit of a road set at 40mph. Car C is a compact car completely hidden from view of Car A as it is in perfect synchronicity of angle behind Car B at the time Car A begins to cross over the northbound lanes. There is an impact between Car A and Car C, Car B not being affected at all and able to continue on, yet playing the part of being able to determine the speed of Car C. Distance traveled by Car B is approximately 1/8 of a mile before judgment by Car A determines it is safe to enter traffic. At impact, Car A is moved about 25 feet forward into the northbound lane by Car C and turned from perpendicular with the oncoming cars to almost parallel. Fortunately injuries are minimal. This is a two-part question: What would be the force at which Car A was hit, thereby determining the assumed velocity of Car C?

Sismart
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  • I'm probably wrong however I don't think you have enough information to determine if car C was speeding or not. If C was still behind car B for that 1/8 of a mile C probably was within +/- 5 mph of car B. Physics aside failure to yield is failure to yield and the court won't care really if C was speeding. – scrappedcola Dec 28 '15 at 16:29
  • This is a partially inelastic collision. The elasticity depends on the type of both vehicles, their orientations, positions, speeds, and even temperature (although I think that effect would be minor). Its highly doubtful you could get any info on that, so it would just be a very rough estimate, and I've no idea what it should be. – Digiproc Dec 28 '15 at 16:33

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There's really no way to determine from the collision how fast C was going. There are too many variables involved to calculate anything exactly; even things like which way A's front tires were pointed can have a noticeable effect.

However, if C was in fact completely obscured by B for the duration in question, then you can use the geometric/trigonometric facts to determine C's speed given B's speed, since you can use the line of sight as a shared hypotenuse of two triangles, the northbound side of which represents the distances B and C travel in an equal time period. This also would imply that C was accelerating during that period, as would be necessary to remain obscured by B from A's perspective to one side of the road.

Asher
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