1. Limited time only! Sign up for a free 30min personal 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 Exam 2012 # 23 (Measuring local gravitational acceleration)

  1. Dec 28, 2012 #1
    1. The problem statement, all variables and given/known data

    Which of the following sets of equipment cannot be used to measure the local value of the acceleration due to
    gravity (g)?
    (A) A spring scale (which reads in force units) and a known mass.
    (B) A rod of known length, an unknown mass, and a stopwatch.
    (C) An inclined plane of known inclination, several carts of different known masses, and a stopwatch.←
    (D) A launcher which launches projectiles at a known speed, a projectile of known mass, and a meter stick.
    (E) A motor with a known output power, a known mass, a piece of string of unknown length, and a stopwatch

    3. The attempt at a solution
    I eliminated A because that is a fundamental way to measure gravity, as a scale reads normal force. Simply place an object on flat ground where Normal force = mg, divide the known normal force by the known mass, and we have gravity.
    I eliminated B since we can use uniform accelerated motion.
    Δy = v_0t + 1/2at^2
    since v_0 = 0
    Δy = 1/2at^2, we also know t, so clearly we can find a
    C I believed you could find, I'm not sure why you can't.
    D I thought you could use projectile motion equations.
    E I'm not sure.
  2. jcsd
  3. Dec 28, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    With C, having carts of several different masses is not that useful, because every mass will take the same time to get from the top of the incline to the bottom. The time depends only on the length of the incline, the inclination angle, and the acceleration due to gravity. So you really only need one cart, but you must also know the length of the incline. Perhaps you are meant to assume that you do NOT know this (because you have no metre stick)?
  4. Dec 28, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    For E, maybe you are supposed to try and wind the string (with mass hanging from it) onto the motor shaft at a constant speed, and since P = Fv, if you know v, you can find F, which is equal to mg (since the speed is constant, the lifting force must be balancing the weight). The thing I don't get is how you're supposed to get v with just a stopwatch and no known lengths. EDIT: Are you sure it wasn't a piece of string of *known* length?
    Last edited: Dec 28, 2012
  5. Dec 28, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    For E, you can use the motor and the stopwatch to deliver a known KE. Knowing the mass that tells you the speed. Use that to launch the mass vertically and time it to top of trajectory.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: F = MA Exam 2012 # 23 (Measuring local gravitational acceleration)
  1. F=MA 2012 Exam #19 (Replies: 8)