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Q & A: Weight Changing Elevators

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Most recent answer: 09/25/2011
Q:
Will a person’s weight be changing when he is in a moving elevator? Will increasing or decreasing the speed of the moving elevator up and down affect the weight of the person?
- Alvin (age 12)
Singapore
A:
We must first make the distinction between weight and "apparent weight". The definition of "weight" is just the magnitude of the force of gravity on an object, which wont change much unless you take the elevator very far from the surface of the earth. What I think you are talking about is how heavy something feels, or "apparent weight", so this is what I will discuss below.

Isaac Newton told us something that will help here. Objects in motion tend to stay in motion unless acted on by something else. That may sound complicated, but an elevator is a great way to see this.

When the elevator is moving, we will weigh our normal weight. Since we are already moving at the same speed as the elevator (up or down), nothing is affecting us to change our weight. However, when the elevator starts to go or stops, our body resists it. Let's say we are going down. The elevator stops. According to Newton, our body wants to keep going, but the floor will not let us keep moving. So for a short time, we are pushed into the floor more and our weight goes up.

The same thing works when the elevator starts moving. Our body is happy to be standing still. But if the elevator starts to go down, our body wants to stay still. That's why you feel light at first when the elevator starts to go down. Once the elevator starts to move, we get used to moving and we don't feel it anymore.

Adam

(published on 10/22/2007)

Follow-Up #1: Change of weight in an elevator?

Q:
By which formula we can calculate our change in weight when it started going up or down???
- Kiran Pachghare (age 26)
Mumbai, India
A:
It all has to do with the total force from the floor on the body inside the elevator.  There are two components: 
1)   the force needed to balance the force of gravity = g*m   where g is the acceleration due to gravity and m is the mass of the body,
and:
2)   the force giving the acceleration  = a*m  where a is the acceleration of the elevator.

The total force is the algebraic sum.   Note that when the elevator is moving at constant velocity the acceleration is zero and you don't feel any difference.

LeeH

(published on 01/20/2010)

Follow-Up #2: Weight inside of an accelerating elevetor.

Q:
A man having a mass of 100kg stands on a bathroom scale while inside an elevator.What is the reading of the scale when the elevator is at: a)rest or moving up or down at constant velocity b)accelerating up at 1.00m/s^2 c)accelerating down at 1.00m/s^2 d)what is the reading when the cable supporting the elevator breaks and the elevator falls freely?
- kristy love balgos (age 15)
iloilo city, philippines
A:
The force from the scale adds to the gravitational force to give the total force on the man. The gravitational force on an object of mass m at rest on the earth is Fg = mg, where g is the acceleration due to gravity.   Any other force due to spacial acceleration is Fother = maother , where aother=a-g, where a is the total acceleration.   These accelerations are vectors so they must be added vectorially, keeping track of direction.

The answer to part (d) of your question depends on the presence or absence of air resistance and friction between the elevator and the walls.  Either one of them will tend to reduce the net downward acceleration.

LeeH (small mods by mw)

(published on 09/25/2011)

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