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Checking Speed/Velocity Porblem Answers

  1. Sep 22, 2013 #1
    1. The problem statement, all variables and given/known data

    I have two problems.

    1. Your dog is sent to fetch the newspaper in the morning. He runs directly away from you,traveling 62.0 feet in 5.0 seconds. He then turns around and in 4.0 seconds runs 30 feet back toward you before deciding to stop. Calculate the dog's average speed and average velocity.

    2. Two cars are traveling 12 miles apart and are approaching each other on a straight road. Each car is moving at 60 million/h relative to the road. How long will it take before the cars pass each other? Give your answer in minutes.

    2. Relevant equations



    3. The attempt at a solution
    I am pretty sure of my answers, but would like to check them before turning them in.
    1. Speed: 10 ft/s Velocity: 32 ft/s

    2. 6 minutes.


    Thanks in advance!
     
  2. jcsd
  3. Sep 22, 2013 #2

    haruspex

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    The speed is about right (don't know how many significant figures are required), but for the velocity you appear to have forgotten to do something.
    The time in q2 is right.
     
  4. Sep 22, 2013 #3
    Thank you! Do you have an idea of what I may have forgotten? For the velocity I subtracted 30 ft. from 60 ft. and then divided by 5 seconds minus four seconds, yielding 32 ft. over 1 second, yielding my answer of 32 ft./s
     
  5. Sep 22, 2013 #4

    haruspex

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    Why did you subtract 4 seconds from 5 seconds? Did time run backwards?
     
  6. Sep 22, 2013 #5
    Oh! Thank you! I assumed that since I subtracted in the numerator I should subtract in the denominator, but I wasn't thinking critically. So I would actually add 4 and 5, yielding an average velocity of 3.5 ft/s.
     
  7. Sep 22, 2013 #6

    haruspex

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    Much better.
     
  8. Sep 22, 2013 #7
    Thank you! That was the one answer that seemed off.
     
  9. Sep 22, 2013 #8
    Wow. I can't believe I misspelled "problems" in this. I should drink more coffee before attempting physics tomorrow.
     
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