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Acceleration upwards and the effect of 'g'

  1. Aug 24, 2013 #1

    If a statement something like this is given:
    "A body is going up with a constant acceleration of 2 m/s^2"

    Does it mean that acceleration due to gravity acts on it and we have to subtract 9.8 from 25?

    We subtract forces by newton's laws of motion right?

    There is a question like this:
    "A lamp hangs vertically from a chord in a descending lift. The lift has a deceleration of 5.2 m/s2 before coming to a halt. If the tension in cord is 30 N, find the mass of the lamp."

    We answer it by T=m(g+a) ; a= acceleration/deceleration and get the answer.
    but here we use g+a .

    why not just acceleration?
    Why shouldn't we subtract or add g in case of 1D motion?
    Last edited: Aug 24, 2013
  2. jcsd
  3. Aug 25, 2013 #2


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    Staff Emeritus
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    Homework Helper

    Where do you get 25 from?

    If a body is accelerating upward at 2 m/s^2, it cannot at the same time be accelerating at 9.8 m/s^2 in the opposite direction.

    In the elevator question, you should draw a free body diagram of the lamp and the cord to determine all of the forces acting on the lamp while the elevator is coming to a stop.
  4. Aug 25, 2013 #3


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    Science Advisor

    Those are two contradictory questions! You ask why we use g at all in the first question then ask why we shouldn't use it in the second question!
  5. Sep 1, 2013 #4
    I am sorry. It's not 25 but 2.
  6. Sep 1, 2013 #5


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    Staff: Mentor

    The force of gravity acts on it. The net force on the body produces the given acceleration of 2 m/s2 upwards. The net force is comprised of the gravitational force and the force exerted by whatever mechanism is propelling the body upward.
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