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How much do you weight in an elevator?

by WRNWRN
Tags: elevator, weight
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WRNWRN
#1
Jan7-12, 11:25 AM
P: 2
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.
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#2
Jan7-12, 11:28 AM
P: 418
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.
rorix_bw
#3
Jan7-12, 11:40 AM
P: 145
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.

bigfooted
#4
Jan7-12, 12:36 PM
P: 284
How much do you weight in an elevator?

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.
alexg
#5
Jan7-12, 03:57 PM
alexg's Avatar
P: 126
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.
daqddyo1
#6
Jan7-12, 04:35 PM
P: 30
Bigfooted and Alex,
I have several concerns about your explanations

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.

Now, think about the readings on the bathroom scale if
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.
D H
#7
Jan7-12, 04:47 PM
Mentor
P: 15,053
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.
Drakkith
#8
Jan7-12, 04:48 PM
Mentor
Drakkith's Avatar
P: 11,473
Quote Quote by daqddyo1 View Post
Bigfooted and Alex,
I have several concerns about your explanations

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

In contrast to this "purely gravitational" definition, some books use an "operational" definition,[citation needed] defining the weight of an object as the force measured by the operation of weighing it (using a force-sensitive scale, such as a spring scale), in vacuum. This is the force an object exerts on a scale, and is equal to the force required to support it (although in the opposite direction to the "weight" force). This force measured by force-scales is the same as what some other sources term the object's "apparent 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.
OCR
#9
Jan7-12, 05:19 PM
P: 124
This might help... the term "acceleration due to gravity" is not really that easy to define.

It's kind of relative.

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

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

http://en.wikipedia.org/wiki/Earth%27s_gravity

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

http://en.wikipedia.org/wiki/Gravita...ral_relativity



OCR
alexg
#10
Jan7-12, 10:42 PM
alexg's Avatar
P: 126
Quote Quote by daqddyo1 View Post
Bigfooted and Alex,
I have several concerns about your explanations

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

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

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


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