Jun 7, 2008

Posted by in Physics Talk | 32 Comments

Random act of science teaching – you weigh less on the way down in the an elevator


Random act of science teaching. These scales were mounted inside commercial and residential elevators allowing riders to test this fact.

Explanation was available as a leaflet in the the elevator.

Everyday physics, tested by everyday person, using everyday objects.

Talk Like a Physicist

  1. Guess Who says:

    Now instead of dropping overweight Americans will live in elevators.

  2. Oh, sure, I weigh less as we start down, but I gain it all back as we travel, and I weigh even more as we stop on the first floor.
    Dang crash diets!

  3. Surely you weigh less only while the elevator is _accelerating_ downward?

  4. Mr. Physics says:

    Um…that scale is completely misleading. You do not weigh any different whether you are moving up, down, left, right, or even accelerating up, down, left or any which way.

    Weight is another name for the force of gravity, and that can only be changed if you change your mass or move to a different planet – both of which do NOT change while in an elevator.

    The scale is actually measuring the force that you exert on it, otherwise known as a normal force (because it is perpendicular to the surface). Typically the normal force that the scale exerts on you is equal to your weight because there is no acceleration. When forces balance, there is no change in your motion.
    However, when you accelerate upward or downward in an elevator, you need the normal force and your weight to NOT be balanced, so therefore the scale can read something different.
    If you are simply traveling downward without any acceleration, the forces are balanced and the normal force that the scale exerts on you is equal to the force that the Earth exerts on you….so you do NOT weigh less when moving downward in an elevator.

    Your apparent weight (the normal force caused by the scale on you) will register less when you are accelerating down.

    Sheesh! Take a physics class before you advertise how good you are at physics.

  5. Mr Physics fails says:

    Actually, “weight” is a measure of how much normal force you exert on whatever the “ground” is, in this case the elevator. So yes, you weigh less when moving down the elevator.

    Sheeesh! Try general relativity before spouting of about elevators.

  6. “Weight is another name for the force of gravity, and that can only be changed if you change your mass or move to a different planet – both of which do NOT change while in an elevator.”

    Gravity is not a force, gravity is an acceleration. The elevator changes the acceleration, so it also changes the gravity.
    Also, gravity does change with elevation, so people farther away from sea level will weigh move.

  7. Alex fails says:

    Gravity is a force. One of the elemental forces, if you want to put it that way. Force makes you accelerate -> acceleration = force

  8. The force will be with you…..always..

  9. Alex fails... fails says:

    force = mass*acceleration
    technically you gain “weight” but your mass stays the same, as stated above the normal force it a measure of the normal force you exert upon an object.

    gravitational acceleration comes into play with 9.82 m/s²
    total acceleration = gravitational acceleration + lift acceleration

    when the lift rises the acceleration used to increase the speed of the lift (moving up and down) initially acts with the 9.82 m/s² from the force due to gravity to increase the over all force, when the lift approaches the correct floor the lift then slows giving it negative acceleration which acts against the force due to gravity.

  10. PohTayToez says:

    The scale is definitely misleading, but not in the way Mr. Physics said. You’ll weight less when the elevator starts accelerating downwards, but then it will even out when the elevator stops accelerating, (reaches a constant speed). Then, when you’re near your floor, you’re start to weight more as the elevator decelerates downward (technically accelerating upward).

  11. Any difference in weight displayed by the scale while the elevator is moving at a constant velocity is likely due to Hysteresis.

    The real question is do bloggers post wrong science just to get nerds to comment?

  12. But don’t you gain mass as you speed up based on e=mc^2, since it means you’d have more kinetic energy?

    More kinetic energy would mean more mass… though I suppose the acceleration would counteract that (even if the scale were sensitive enough to perceive the additional .*lots of 0s*1 grams you gain.. seeing as we’re talking about weight.. not mass.

  13. Christopher says:

    Ooo wow everyone, look how intelligent I am. I took a high school physics course and as such I can nitpick this article like a pedant, instead of remarking on the point of the scale, which is to encourage scientific interest and learning. Which is something a brilliant internet physicist such as yourself should appreciate.

  14. Its still wrong, chrispopher……

    I would argue that your weight does change.

    Becuase if you are going down, you are by definition getting closer to the center of the earth.
    We all now gravity is apporxiamtly proportinal to the square of the distance between the objects….
    So if we are getting close, gravity is excerting a deffernt acceleration on us, thus our weight changes.
    Maybe by inly a litte, but none the less.

    And for the record, weight is a mesurement of the gravitaional force (yes, force) acting on an object.
    Not any old force, but gravity.

    So unless the lift screws with gravity in some way, it does not change your weight.

    Mr physics fails fails

  15. People please, mass is constant, even on the moon your mass is the same as it is on Earth. What is variable is your weight. Your weight changes as the elevator accelerates up and down the shaft. Ok? (Physics major in University for your information)

  16. A different Zak says:

    Other Zack is right. Mr. Physics gave a semantic argument which is equivalent to the original argument. Rest mass is an intrinsic property of an object. The observed force acted on you by the ground (commonly called weight) is dependent on how you are accelerating. Hence, your weight changes as the elevator moves.

    Mr. Physics is a perfect example of why people who don’t think with math can never really understand physics.

  17. @ Zack, I don’t think anyone said your mass would change, they were arguing as to whether or not your weight changes due to the movement of the lift.

    I would be inclined to say that, yes, your weight does change, but only as the lift is accelerating. When the speed of the lift is constant, the normal force exerted on you by the lift floor will remain constant and so therefore your weight will remain constant, as weight is a resultant of the normal force exerted on you by the surface you are standing on.

    However as the lift is accelerating the forces exerted on you will not be greater or smaller depending on whether the acceleration is positive or negative and that in turn will change the normal force exerted on you and so therefore your weight will not be the same.

  18. *EDIT to previous post: I meant to say “the forces exerted on you WILL be greater or smaller depending on whether the acceleration is positive or negative”

  19. This whole is a “strange” effect of the Newtonian mechanics. When you try to “measure” a force, weigth of an object in our case, you have to take into the account the observer in the calculations. If the one that makes the observation is the one in the elevator he will see his weight change, due to the fact that because he is moving alongside with the frame of reference you only can perceive that the acceleration of gravity [g] changes (because of the elevator’s acceleration, but nevertheless you cannot discriminate between this change and an actual change in g). If the observation had been made from the outside of the elevator, therefore using the newtonian inertial frame of reference the count of your weight would have been the same.
    This whole problem is widely known in mechanics as the problem of the elevator (or something like this…!) and it is used to make clear that there is no difference between the weight of an object because of acceleration and the weight of the object because of gravity. It was also one of the problems that led to the improvement of the Newtonian mechanics by Lorentz.

  20. little bunny foo foo says:

    you weigh less on the way up because the force of gravity on your body is dependent upon the distance between you and the center of the earth. The closer you are to the center of the earth the more you weigh.

  21. Hey, this is from Science World, in BC, Canada!
    I used to love going there on field trips and such, the ball tower is so cool! I <3 contraptions!

  22. It’s not true that a person weighs less when coming down an elevator than when going up. You weigh less at the BEGINNING of a trip down than at the BEGINNING of a trip up,and MORE at the end of a trip down than at the end of a trip up.

    At the beginning of a trip down, the elevator is ACCELERATING
    downward. Soon it reaches a steady speed, and your weight becomes normal. Then it DECELERATES downward (which is the same as accelerating upward) as it comes to a stop at the final floor. Likewise, when going up the elevator first accelerates upward, then reaches a steady speed upward, and finally decelerates to a stop.

    If you stand on a scale in an elevator, there are two forces acting on you: the downward force of gravity, which is constant (for the purposes of this discussion), and the upward force of the scale on your feet.

    When you are stationary or when you are moving at a constant speed, these forces are equal and opposite and the scale shows your correct weight. But when you are accelerating up or down, the effect of that acceleration changes the net force and the scale shows a different weight.

    Net force is calculated by mass (your actual weight) times
    acceleration and is written as ‘ma’.

    Thus, to pick one case, at the beginning of a trip up in the elevator, you are accelerating upward with acceleration “a”. The vertical force equation is

    N – mg = ma

    where N is the upward normal force of the scale and -mg is the downward gravitational force. We can solve for N:

    N – mg = ma
    N = mg + ma
    N = m(g + a)

    The scale measures the force N, dividing it by g (gravity) to read in units of mass (pounds in this example):

    N/g = m(g + a)/g = m(1 + a/g) = m + ma/g

    You can see that the scale reads higher than your normal “weight” m because “a” is positive (m and g are always positive). On the other hand, at the beginning of a trip down in the elevator, the scale reads lower than your normal “weight” because “a” is negative (accelerating downward).

  23. E =mc2
    Any way you look at it. You even weigh more on the way up.

  24. What do you guys think, is it a Miata or a Del Sol?

  25. mass and weight are two different things.

  26. Acceleration has nothing to do with it, all of you have been out of school for to long

    the weight of an object is the magnitude, W, of the force that must be applied to an object in order to support it (i.e. hold it at rest) in a gravitational field.

    If you are slowly moving downward The forces pushing up from the elevator are less than the force needed to keep you at rest. example if you weigh 170 lbs and are moving downwards then we know the elevator isn’t pushing you upwards with 170 lbs of force. it is pushing with a number < 170 lbs which would be measured by a weight scale as a drop in weight since you are falling and if you are going up; Again with an acceleration of 0 but velocity of any number the floor is pushing up on you at a force greater than 170 lbs making you rise against the gravitational pull. it’s all in the definition please refresh your memories of basic equations, you might build or work on something I could be injured by =(

    (( Kept it simple ))


  27. what does it matter says:

    what does it matter, you are an obese american no matter what your scale tells you!!

  28. Agreed, “what does it matter”.

    Stumbled this conversation, and (unfortunately) I have to cast my vote for Mr. Physics. Though stuck-up, he is correct about both the definition of weight and the fact that your weight does not significantly (your weight may increase slightly as you move closer to Earth) change in an elevator.

  29. lol what a bunch of nerdz

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