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Homework Help: Please help - Gr. 11 physics question about net force calculation

  1. Apr 24, 2014 #1
    So this thread has been closed and I can't comment on it. I am working on this question as well and am stuck on b). I managed to solve for a) and c) but I have no idea how to go about finding the force normal and do not know where the formula "Fnet= mg + ma" came from or why to use it. Could someone please break this down and explain it? Physics is super confusing and frustrating. :cry:

  2. jcsd
  3. Apr 24, 2014 #2
    Assuming usual notation is in use here (g = acceleration due to gravity, m is the mass of the object, a is another acceleration, possibly not parallel to the direction of gravity, Fnet is the net or total force acting on the body).
    The equation is just stating that the total force acting is the sum of the force due to gravity (mg) and that causing the acceleration a(ma). mg and ma by Newton's second law (force = mass X acceleration.)
  4. Apr 26, 2014 #3
    Thanks for the response.
    Okay, that makes sense. Also, is ma being subtracted because gravity is [down] but the elevator is accelerating [up]? So that it would make ma [down] now that there is a negative?
  5. Apr 26, 2014 #4


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    Staff: Mentor

    The force delivered by the cable does two things: it supports the weight of the elevator + causes an acceleration of the elevator.

    So, the force component that delivers the acceleration = (strain in cable) - (weight of elevator)
  6. Apr 26, 2014 #5
    yes the 360 N for part b) is correct. Drawing the free body diagrams helps keep the signs of the terms correct. For the elevator the forces are (assuming up is positive) cable tension up (positive), weight of elevator + passengers down (negative), and friction force on elevator down (negative) because it opposes the elevators motion up. Newtons' second law sums those forces equal to the (elevator + passengers masses) multiplied by the acceleration (up) of the elevator including passengers . Your answer of 0.49 m/s^2 is correct for a). Your answer for c) is also correct. The free body diagram for the 35 kg passenger is his weight pulling down (negative) plus the force due to the elevator pushing up on his feet (positive). Newton's second law on his body: his mass times his acceleration up (+0.49 m/s^2) equals his weight (-35 kg * 9.8m/s^2) plus the force the elevator is pushing up (positive on his feet). It is this last term that you are looking for, solving for this force gives 360 N. You see he, that is his feet, feel his weight plus the incremental force caused by the elevator's acceleration up. If you have any further math, physics, chemistry, etc problems, you can e-mail me at njoysci@outlook.com
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