Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Elevator and momentum

  1. Jan 1, 2008 #1
    1. The problem statement, all variables and given/known data
    Moving at a constant speed, an elevator lifts a 1200N package 10m in 3 sec. During this time, the impulse delivered to the package by the elevator floor is
    a) zero
    b) 12000 Nm up
    c) 3600 Ns up
    d) 12000 Ns up

    2. Relevant equations
    change in momentum = net Force*time interval

    3. The attempt at a solution
    zero b/c if it's moving at constant speed, the acceleration is zero. therefore, net force = zero.
  2. jcsd
  3. Jan 2, 2008 #2

    Shooting Star

    User Avatar
    Homework Helper

    There are two forces acting on the body -- reaction force of the elevator floor acting upward, and the weight of the body acting downward. The net force and the net impulse is zero, but the question specifically asks for the impulse delivered to the package by the floor, and thus should be 3600 Ns up. (Note that impulse is a vector quantity.)
  4. Jan 3, 2008 #3
    Thanks ShootingStar. You've been a great help. :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook