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Solving one-dimensional Motion Problems

  1. Sep 8, 2013 #1
    I'm having a little trouble with the wording in this problem. (And this isn't the first time).
    My instructions are:
    "Draw a pictorial representation of each situation described below. That is, (i) sketch the situation, showing appropriate points in the motion, (ii) establish a coordinate system on your sketch, and (iii) define appropriate symbols for the known and unknown quantities. Do not solve.

    a) A car traveling at 30m/s screeches to a halt, leaving a 55m long skid mark. What was the car's acceleration while braking?

    So am I solving the cars acceleration or not? So far I've gotten my known as the velocity 30m/s, my unknowns would be displacement and acceleration, and a very rough picture of a car and an arrow showing forward motion with 30m/s under it and then a line indicating the 55m skid mark. I have no idea what it's really asking me to do though. Do I solve it? (It says not to be then asks a very specific question.) I'm so confused.
     
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  3. Sep 8, 2013 #2

    Redbelly98

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    Welcome to Physics Forums :smile:

    Looks like you are not to actually solve the problem. Even though it proceeds to ask for the acceleration, which I agree is confusing. It sounds like your teacher (or professor) wants people to get practice with drawing sketches to set up a problem.

    Well, I don't agree 100% with your knowns and unknowns. There are typically 5 quantities to worry about in a problem like this. Hopefully you are already familiar with them:
    • a the acceleration
    • vi the initial velocity (some people use u instead of vi)
    • vf the final velocity (some people just use v for this)
    • x the displacement (some people use s, some use Δx instead)
    • t the time
    I recommend doing the following, when you are trying to solve a problem like this:

    1. Look at every quantity given in the problem statement, and figure out where in the above list it belongs.

    2. Look at what is being asked for in the question (acceleration, in this case). It should be in the above list. That is your unknown.

    3. Make sure you have 3 knowns. Well, you might have 4. But you really need 3 in order to solve this type of problem***. If you don't have 3, you are probably missing a verbal clue in the problem statement that tells you one of the quantities is zero.

    Regarding #1 above: Your problem statement has the values "30 m/s" and "55 m". You have incorrectly assigned "30 m/s" as "the velocity", without specifying if that is the initial or final velocity. Also, you have not figured out how the "55 m" is relevant. While it's possible that a problem statement might provide unneeded information, that is not the case here: the 55 m is needed -- or would be needed, if you were actually going to solve the problem.

    Regarding #2: I agree that acceleration is an unknown.

    *** Exception regarding #3: this does not apply when you have a constant-velocity problem. But it does apply here, since velocity is not constant.

    Hope that helps.
     
  4. Sep 9, 2013 #3
    It very much did help thank you so much. I'm glad I found these forums lol.
     
  5. Sep 10, 2013 #4

    CWatters

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    Basically it's just asking you to draw and label the diagram that you would use to help you solve the problem in (a) but you don't need to actually solve the problem.

    It's because too many people rush to solve problems without drawing a suitable diagram and without defining the variables used in their equations.
     
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