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Displacement question

  1. Oct 8, 2008 #1
    1. The problem statement, all variables and given/known data

    A police car joins a straight motorway at Juction 4 and travels for 12 km at a constant speed for 400 seconds, then it leaves at junction 5 and rejoins on the opposite side and travels for 8 km at a constant speed for 320 (s) to reach an accident.

    Calculate the displacement from junction 4 to the accident.

    2. Relevant equations

    No idea, I really need explanation on how to work this out, a link and other practice questions would be really helpful.

    3. The attempt at a solution

    I have not attempted it, but I know that with displacement graphs, deceleration is a downwards curve and acceleration is a curve with increasing gradient. When an object reverses its velocity changes, however I see no relevance of this to my question.

    Please HELP! :confused:
  2. jcsd
  3. Oct 8, 2008 #2
    Well from the starting point, the car travelled 12 km in one direction (and as I understand it) it then travelled 8km in the opposite direction. So if you take the first direction as positive, then the opposite direction would be a negative direction.

    Therefore the car first travelled +12km and then -8km.

    Just sum up the journeys to get the displacement in this plane.
  4. Oct 8, 2008 #3
    OK so just simply 12 - 8 = 4 km

    OK, so next question is to sketch a displacement-time graph for it.

    So, I know the velocity on each side of the motorway is 30 m/s and 25 m/s in the opposite direction. So how do I sketch the graph ?

    Does I make a table of values or something ?
  5. Oct 8, 2008 #4
    Not necessarily. From the name you know that one axis will be displacement and the other will be time.
    Time is the independent variable so by nature you'd put it on the x axis.

    When you think about it a little, you can see as time goes on the displacement (from the origin) will increase linearly (constant velocity) and then when it turns around, it will decrease linearly.

    The rate of increase/decrease (gradient) is rise over run. When you divide rise (displacement) by run (time) you will see that you get velocity.
    So your gradients will be 30 and 25 respectively.
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