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Homework Help: Help me out with these speed, velocity and momentum related questions?

  1. May 27, 2012 #1
    1. The problem statement, all variables and given/known data
    Two cars collide and go off in different directions.

    Car 1:
    Mass = 1010kg
    Distance before collision = 24m
    Distance after collision = 11.5m
    Angle from normal = 55.5°
    - Car 1 was stationery immediately before impact.

    Car 2:
    Mass = 1230kg
    Distance before collision = 22m
    Distance after collision = 18.6m
    Angle from normal = 35°

    μ = 0.55 (for both cars).

    2. Relevant equations
    1. Determine the speed of each vehicle immediately after impact as they slide to their final positions.
    2. Determine the speed of the vehicles immediately before the collision (Law of Conservation of Momentum).
    3. From the skid marks (distance before collision) find the speed of the vehicles before braking.
    4. Calculate the distance and time from the point of impact before each driver recognised the imminent danger.

    3. The attempt at a solution
    I'm thinking perhaps it means velocity rather than speed, that makes more sense to me. In that case, I would use equations of motion for 1 and 2, maybe 3 as well. However, initially I thought to use instantaneous velocity for question 1. Question 4, I have no clue, except that "before each driver recognised the imminent danger" is referring to the skid marks again.

    Any help will be greatly appreciated!
    Last edited: May 27, 2012
  2. jcsd
  3. May 27, 2012 #2

    Simon Bridge

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    Nah - speed is fine. Think: difference between speed and velocity.
    Only for 1 and 3 - like it says, use the conservation of momentum for 2.
    What changed your mind?
    yep - that refers to the skid marks.

    1. kinematics - can get the acceleration off F=ma.
    2. you need to use the angles to work out the initial speed of car #2 since #1 starts out stationary.
    3. same as 1 but for before the collision.
    4. the way I read this one is that you are being asked to factor in the reaction time for the drivers to notice they are in trouble to when they actually slam on the brakes.

    note: double post
  4. May 27, 2012 #3
    That makes a lot of sense. We were also given reaction times, if I recall correctly.
    Thank you so much, you're a life saver. ^^'
  5. May 27, 2012 #4
    Maybe you furnish graphic presentation of the vehicles after and before the collision.
  6. May 30, 2012 #5
    I'm confused. How do I use the Law of Conservation of Momentum if I don't have the time (how long they drove before the crashed, and the time it took for them to stop)? :S
  7. May 30, 2012 #6

    Simon Bridge

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    for (1)

    length of skid is d, acceleration (you have to find this) is a, time to slide to a stop is t (unknown) and initial speed is v (also unknown).

    the v-t graph is a triangle:
    displacement is the area under the graph: d=vt/2
    acceleration is the slope of the graph: a=v/t

    that's two equations and two unknowns.
  8. Jun 3, 2012 #7
    Your correct it wants velocity for q1 & q2

    your given the coefficent of friction which is important for your motion equations.
    Coefficent = Ff /Fn

    Q2 the conservation of momentum is probably the easiest part and theirs tons of examples in the class work and in your text book.

    Q4 its basically asking for the total time from when they saw the other car and the distance.

    need anything else facebook me
  9. Jun 3, 2012 #8
    The Momentum Part is actually very simple because it wants you to find the initial velocity right and momentum is velocity * mass. so the combined momentum of the Car and the 4wd must equal the original momentum of the 4WD

    Pi = Pa + Pb
    In this rule Pa and Pb are vectors so some trig will have to be done to work it out depending on where the car's are.
  10. Jun 3, 2012 #9
    alright umm thats a tough one but
    q1 you work out the speed immediately before impact

    I'll check your answers at school.
  11. Jun 3, 2012 #10

    Simon Bridge

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    @Veloco-raptor: how did you get on?
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