1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

F = MA 2012 Exam # 19, 20 (Using Graphs)

  1. Jan 26, 2013 #1
    Woops, it's the 2010 exam -- my bad.


    1. The problem statement, all variables and given/known data
    They have diagrams that I'm not sure how to display; so find them at the following link:
    https://aapt.org/physicsteam/2010/upload/2010_Fma.pdf
    These are #'s 19 and 20 and they refer to the PE graph in 18.

    2. Relevant equations
    Not sure, perhaps:
    E_mec = K + U

    3. The attempt at a solution
    I'm not at all sure how to use the PE graph and a position vs time graph to find total energy or describe the motion. Also, I'm not sure how a potential energy vs position could give rise to position vs time. If anyone could direct me to the concept or link that I'm missing -- that'd be great.
     
  2. jcsd
  3. Jan 26, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Use the graphs separately.
    What is the motion of the particle in the potential?
    (You did Q18 OK didn't you? So you know the forces?)

    i.e. Will the particle have a constant velocity or just hold it's position, or accelerate or what?

    Then look at the position time-graphs. What kind of motion do each describe?
    Which one matches up?

    You can do it the other way around ... look at the first graph: what is the particle doing? What happens to it's position?
    Now look at the potential vs position graph - does that make sense? (This is probably easier.)

    For Q20 - it is pretty much the same ... what is the motion of the particle? Describe it in words - is it sitting still, moving at a constant velocity, oscillating between limits, what? At what kinetic energy would the particle have t have in order to have that motion?
     
  4. Jan 26, 2013 #3
    So for numeral I, it makes sense since net force is 0 at 15m so acceleration is zero and this can be represented by a constant position. TRUE
    For numeral II, there should be a positive acceleration but the position vs time graph is constant; clearly FALSE.
    Numeral III, the velocity is constant - thus the acceleration is zero. This agrees with the graph. TRUE
    So 19 is both I and III. Thanks

    Here's a thought for 20:
    E_mec = K + U
    E_mec = 1/2mv^2 + U
    Let's make it easy on ourselves by finding where v = 0 (where dx/dt = slope = 0)
    Two points of this are: x = 5 m and x = -5m
    Here K = 0 so
    E_mec = U
    Tracking these points on the graph both yield:
    -5J

    Wow, those questions were actually kind of easy; thanks for your insight. I guess I should really pay more attention to what I am given.
     
    Last edited: Jan 26, 2013
  5. Jan 26, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Reading graphs is a skill that many people resist learning because it looks harder than it is. It is actually a good shortcut to solutions.

    i.e. #20 the graph shows the particle oscillating sinusoidally between -5 and +5 ... to find the kinetic energy that does that, draw a horizontal line through the potential energy graph and see where it intersects.

    All the difficult part is in knowing which lines to draw.
    Anyway - well done.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook