1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Car Collision - Which car was speeding?

  1. Jan 10, 2012 #1
    This problem is an equation derivation question. An equation needs to be derived. This equation is then used to make a procedure to find the required variables (You are an engineer onsite).

    1. The problem statement, all variables and given/known data
    Two cars collide on a road and leave skid marks on the road. They 'hook bumpers' and move as one. Derive and equation to calculate their initial velocities to find the speeding car.

    2. Relevant equations

    m1v1 + m2v2 = m1v1' + m2v2 '

    3. The attempt at a solution
    Key Assumptions:
    1. The net force on the cars is zero.

    m1v1 + m2v2 = m1'v1' + m2'v2 '

    Since the cars move as one, we can put the speeds of both cars together:
    m1v1 + m2v2 = (m1 + m2)(v')

    Now I am not sure where to go from here because of many missing variables. The mass of the cars can be found by weighing them. Not sure about the final velocity, you just have skid marks on the road.

    Any help would be appreciated. Thanks!
  2. jcsd
  3. Jan 10, 2012 #2


    User Avatar
    Homework Helper

    The bit that you have done so far is correct. But obviously this is not enough to tell us which car was speeding (if any). In any case, what you are given is the skid marks, and this might not directly give you the final velocity anyway. The information about the skid marks will give the answer, but you haven't said anything about what the skid marks are like. To start the problem, you've got to be clear on what the question says about the skid marks.
  4. Jan 10, 2012 #3
    I wish I could be more specific but our teacher has not given us the question sheet. He simply read it out to us to give us hints as to where to focus on studying.

    I assume the skid marks are created after the crash and can be used to represent the displacement of the cars after collision.
  5. Jan 10, 2012 #4


    User Avatar
    Homework Helper

    maybe your teacher was just getting you guys to start thinking about this kind of problem. In this case, I'd say that the equation that you have given is the important equation for this situation. (Also need to consider that the cars will have motion in 2-d, so the equation will be a vector equation).

    If the skid marks are created after the crash, you are totally right that they will represent the motion after collision.
  6. Jan 10, 2012 #5
    Can you lead me in the right direction? I've been trying to derive the equation but no luck. I'm still stuck with two variables. In this case I know I will have to derive another equation and sub it into the first. Substitution method is the only way I can think of.

    I can't seem to see what the second equation would be. Would the use of the Ek = Ek' equation help? It also has velocity values present and might be possible to use those to isolate to only one equation.

  7. Jan 10, 2012 #6
    If you have the initial velocity and the length of the skid mark you can find the deceleration which will help you with the final speed.
  8. Jan 10, 2012 #7
    Hmm. The problem is what we are not given any values. All values have to be found on our own by assuming we are working as an engineer on the site. So we have to let the teacher know what we will do to find the value.

    For example, mass is not given but I can say, "The mass can be found by weighing the cars".

    I've thought about it a bit more and think the conservation of energy equations must be used. Since it will be a completely elastic collision (cars hook bumpers and move together) it will probably be something like so:

    (1/2)(m)(v)2 = FfΔd

    Now I am confused because this is the energy of the system. I am assuming this velocity would be of the cars when they move together.

    After which I was thinking of isolating v in the above equation and subbing it back into the momentum equation to just have two missing variables (v1 and v2). Like BruceW said it should be in 2D so therefore I can use substitution to just get one variable. I think that makes sense but would love your opinions and ideas.
  9. Jan 10, 2012 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    There are skid marks. Suppose you can measure the skid marks. --- that should give three more pieces of data.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook