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

Weight in an elevator, at terminal velocity

  1. Jun 7, 2012 #1
    There's a discussion on reddit about an elevator not just in free fall, but at terminal velocity. I know that at the moment the elevator starts to fall, you would feel weightless. But does this hold true for terminal velocity as well? You have stopped accelerating, so you are experiencing a pull downward by gravity just as you would standing on the ground. Does the scale read your weight? The consensus is yes, but I can't help but think you would feel weightless.
     
  2. jcsd
  3. Jun 7, 2012 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Ignoring the slight change in gravitational acceleration with altitude, the scale would read the same weight it would read when you stand on it on the ground. Draw a free body diagram.
     
  4. Jun 7, 2012 #3

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The laws of physics are the same in all inertial* reference frames. So it would be impossible to distinguish being in this box falling at a constant speed (relative to the Earth's surface) from being in a box that is stationary (relative to the Earth's surface). In fact, you could claim with equal validity that you were stationary and that the Earth's surface was moving up towards you. This is a key idea behind the special theory of relativity.

    So, no. You would not notice anything different in this elevator falling at a constant speed than you would in an elevator sitting on the ground.

    *non-accelerating, basically
     
  5. Jun 7, 2012 #4
    The air is exerting a force that is equal and opposite to the force of gravity exerted on the elevator. However the speed of the air INSIDE the elevator RELATIVE TO YOU is so small that it would be unable to provide enough air resistance to stop your acceleration. As a result, the elevator stops accelerating, but you don't, so you eventually will hit the floor and weigh in.

    Hope that made sense.
     
  6. Jun 7, 2012 #5
    Got it, thanks.
     
  7. Jun 7, 2012 #6
    I understand initially everyone would be flloating in the elevator like astronauts in an orbiting space capsule.But is an orbiting space capsule considered accelelerating even though its orbital speed is constant? Is this why it has zero gravity?
     
  8. Jun 7, 2012 #7
    Yes, the capsule is accelerating. Any object that moves in circles is accelerating; remember, acceleration perpendicular to velocity doesn't increase the velocity but it does change the direction. This is exactly what happens with circular motion and orbits.
     
  9. Jun 7, 2012 #8
    Nobody is going to float like astronauts in an elevator because an elevator's design makes it physically impossible to free fall. The counterweight typically weighs a little more than the elevator, so if all control mechanisms were to let loose, the counterweight would fall down and the elevator would rise. So until it found its terminal velocity, you would weigh more than normal and not less.

    When you descend normally in an elevator, the control mechanism is hoisting up on the counterweight. But it is possible to overload an elevator, such that its total weight exceeds that of the counterweight by a little bit. In that case, if all control and holding mechanisms were to fail at the same time, you would descend slowly.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Weight in an elevator, at terminal velocity
  1. Terminal velocity (Replies: 1)

  2. Terminal Velocity ? (Replies: 5)

  3. Terminal velocity (Replies: 9)

Loading...