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Homework Help: Two Basic Midterm Questions

  1. Oct 15, 2015 #1
    • Member warned to post individual problemss in separate threads and to show a solution attempt.
    1. The problem statement, all variables and given/known data

    The first-
    A ball falls off an edge 1m high and lands 0.9m away. What is the velocity of the ball as it falls off the edge, and what is the velocity of the ball as it hits the ground?

    The second-
    A man is driving at a constant speed of 20m/s. He sees a deer 112m away and breaks to a stop on its position. His reaction time is 0.5s. What is the car's acceleration as the man brakes? How long does it take him to reach the position?

    If you could answer any part of either problem I would very much appreciate it.
    2. Relevant equations
    (Not all may be needed.)
    3. The attempt at a solution
    Answers to the first- 1.99m/s and -4.43m/s

    Answers to the second- -1.79m/s^2 and 10.67s ( maybe 11.17s)
    Last edited: Oct 15, 2015
  2. jcsd
  3. Oct 15, 2015 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Also, 'break' and 'brake' are two different things.
  4. Oct 15, 2015 #3


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    Science Advisor

    Why are you not showing what you have tried? The concept should be fairly simple- knowing the height, you can calculate the time to fall under the influence of gravity. Knowing the time and horizontal distance move, find the horizontal component of velocity.

    For the second problem, assuming acceleration "a" use one of your given equations to find the time to speed 0 and the distance traveled in that time in terms of "a". Then find the value of a so that is no more than 112 m.
  5. Oct 15, 2015 #4
    Lol, I had to post this in between classes and was kind of burned out by the midterm.
  6. Oct 15, 2015 #5
    I was hoping someone could fact check the answers. Being that I'm a beginner to the subject, I'm not sure my solving is correct.
  7. Oct 15, 2015 #6


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    Staff: Mentor

    The first problem statement is ambiguous. If an object simply falls off an edge it should fall straight down with no horizontal displacement. If it has some initial velocity then the details must be given: what is the initial trajectory? Is it moving strictly horizontally before leaving the edge, or is it launched in some other direction?

    Please quote the original problem statement precisely.

    You should show your problem solving attempts so that you can receive feedback on your methods, not just your results. Confirming or denying a particular numerical result alone is not diagnostic; a simple error in calculation could render an otherwise flawless analysis incorrect. The answer "That is wrong" doesn't really give you a clue as to what actually went wrong, and "That is correct" doesn't address the reason why you had a doubt in the first place.
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