How much do you weight in an elevator?

1. Jan 7, 2012

WRNWRN

Does your weight change in elevator? I have recently found an answer to this question but I can't understand it completely. So, here is the answer : No, your weight is unchanged. To provide the acceleration upwards, the floor or scale must exert on your feet an upward force that is greater in magnitude than your weight. It is this greater force that you fell, which you interpret as feeling heavier. The scale reads this upward force, not your weight, and so its reading increases. I can't understand how the force exerted by the floor on one's feet acts on the scale.

2. Jan 7, 2012

rollcast

Weight is the force acting upon an object due to the acceleration of gravity.

Going up or down in an elevator does not change the acceleration of gravity of therefore your weight does not change.

However your apparant weight will change.

3. Jan 7, 2012

rorix_bw

If you are having trouble visualising this, imagine someone is underneath you and the weighing scale, and they are pushing the scale up towards your feet. So it will read a higher weight because it's being pressed upwards against your feet. Also, you and the scale are lifted off the ground.

That imaginary person is the floor of the elevator.

(If you need to understand how a bathroom weighing scale works, squeeze it in your hands)

Edit: Note that Einstein's elevator says you can't tell the difference between gravity and an equal strength acceleration, so think about what 'weight' means.

Last edited: Jan 7, 2012
4. Jan 7, 2012

bigfooted

we always made the distinction between mass and weight:

http://en.wikipedia.org/wiki/Mass_versus_weight

Mass does not change when acceleration acts upon it, weight does.
This is why you can be 'weightless' when the elevator moves down with an acceleration equal to g, but you cannot become massless.

5. Jan 7, 2012

alexg

It really depends on your definition of weight.

Weight is the force needed to support mass in a gravitational field. But GR tells us that a gravitational field and accelerated motion are equivalent, so the force exerted on a mass by acceleration should also be considered weight. Therefore, your weight does increase in an elevator accelerating in a direction opposite the gravitational field.

6. Jan 7, 2012

daqddyo1

Bigfooted and Alex,

a) Weight is the DOWNWARD force exerted by gravity on a mass. It is never an upward force.

b) Weight never changes unless the mass changes or the FORCE of gravity on the mass changes.

c) When standing on a bathroom scale, gravity acts downward on you so that your weight is an action force acting downward on the scale. The upward reaction force exerted on you by the scale spring balances you . [It prevents yoiu from falling to the centre of the earth.] The scale reading gives you the value of this reaction force and is equal in magnitude to your weight but opposite in direciont.
c) If you are standing on the same bathrrom scale in an elevator at rest or moving at constant speed, all the forces mentioned in b) are the same.

d) Remember that an elevator accelerates for only a short time while it gets up to speed or while it slows to a stop.
Pretend you are standing on a bathroom scale in the elevator. The reading on the scale will tell you the force acting upward on your body.

e) If the elevator is moving upwards and accelerating upwards, the forces acting on your are 1) gravity down and 2) the upward force exerted on you by the scale. Since you are accelerating upward, the upward scale force must be greater in magnitude than your weight. Therefore you will see a scale reading greater than your normal weight reading.

i) you are moving upward and the elevator is slowing.
ii) you are moving downward and the elevator is slowing.
iii) your are moving downward and the elevator is speeding up.

7. Jan 7, 2012

D H

Staff Emeritus
alexg is exactly right. The answer depends on what one means by "weight". Some texts define it as mass times gravitational acceleration, others as what an ideal spring scale measures.

8. Jan 7, 2012

Staff: Mentor

From wikipedia: http://en.wikipedia.org/wiki/Weight

By your definition you would still have weight while in orbit. I think both views are correct, and you need to account for both when applicable.

9. Jan 7, 2012

OCR

10. Jan 7, 2012

alexg

No, weight is the upward force preventing an object from moving downward in a gravitational or accelerated field.

Or the acceleration changes.