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!

Newton's Law on an elevator

  1. Sep 24, 2008 #1
    1. The problem statement, all variables and given/known data

    "A student whose weight is 500 Newtons stands on a scale in an elevator and records the scale reading as a function of time. The data is shown in the graph above. At time t=0, the elevator is at displacement x=0 with velocity v=0. Assume that the positive directions for displacement, velocity, and acceleration are upward.

    Graph (scale reading 'newtons' vs time):
    0-5 seconds --> 500 Newtons
    5-10 seconds --> 700 Newtons
    10-15 seconds --> 500 Newtons
    15-20 seconds --> 300 Newtons"

    3. The attempt at a solution

    a) Free Body Diagram w/ all forces on the student at t=8

    Normal Force going up
    mg going down

    b) Calculate the acceleration of the elevator for each 5 second interval

    0 m/s^2 because force stays the same meaning its at rest since velocity is 0 at 0seconds

    a= 3.92m/s^2

    force is back to 500 meaning constant velocity
    a= 0m/s^2

    a= -3.92 m/s^2

    c) It asks for velocity when t= 5,10,15,20

    v= 0 m/s

    v= 19.6 m/s

    v=19.6 m/s
    v=0 m/s

    Now I have to plot this on a velocity vs time graph so I have it at:
    0 velocity from 0-5s
    line from (5,0) to (10,19.6).
    horizontal line from (10,19.6) to (15,19.6)
    line from (15,19.6) to (20,0)

    Just want to see if I am doing this right... Thank you.
  2. jcsd
  3. Sep 24, 2008 #2
    Just from a quick skim, it looks like you're doing it correctly.
  4. Sep 24, 2008 #3
    thank you.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Newton's Law on an elevator
  1. Elevator Newtons Laws (Replies: 11)