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Trying to understand how to make Motion Diagrams

  1. Jan 26, 2012 #1


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    Trying to do my first Physics home work assignment for college and I'm not really making any headway. He went thru the martial so fast while throwing in calculus that I'm not sure if I do or don't need.
    My main problem with Motion Diagrams is I'm not sure how accurate I'm supposed to be and how do I get that level of accuracy I need?
    The problem I'm on bellow, is really getting me because I don't know how to start. Should I show 35 meters of movement and the reaction time before the deceleration?

    1. The problem statement, all variables and given/known data
    "The driver of a car traveling at 35 m/s suddenly sees a police car. The driver attempts to reach the speed limit of 25 m/s by accelerating at 2.5 m/s^2. The driver has a reaction time of 0.55 s. (The reaction time is the time between first seeing the police car and pressing the brake.)

    2. Relevant equations

    3. The attempt at a solution
    I really don't know how I would show a solution on here.

    Hope my problem makes sense.
  2. jcsd
  3. Jan 26, 2012 #2


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    Staff: Mentor

    Welcome to the PF.

    I would start this problem by drawing a graph with velocity on the vertical axis and time on the horizontal axis. Draw the 35m/s initial velocity starting before time t=0 (to the left of the vertical axis), and then at t=0, that's when he sees the police car. But there is reaction time before he starts decelerating, so the 35m/s line continues for a bit, and then starts decreasing due to the deceleration...
  4. Jan 26, 2012 #3
    I recommend ignoring the reaction time of the driver and just starting by calculating how long it will take for the car to decelerate (or accelerate backwards) from 35 to 25 m/s with the acceleration given in the problem. Do you know how to do that?
    edit: If you can do that then move on to berkeman said above.
  5. Jan 26, 2012 #4


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    Thanks. I think I have a handle on things now. Youtube helped some as well.
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