Question

You ride on an elevator that is moving with constant downward acceleration while standing on bathroom scale. The reading on the scale is
A) equal to your true weight, mg
B) More than your true weight, mg
C) Less that your true weight, mg
D) could be more or less than your true weight, mg, depending on the magnitude of the acceleration

Answer 1

What if I could show you the answer? :P

Answer 2

https://www.youtube.com/watch?v=iuUghXPLeuw

I performed this quite a while ago.. take a look

Answer 3

awsome

Answer 4

thanks

Answer 5

wait but why does weight decrease

Answer 6

Your weight, is determined by how much force the ground puts on you (we call that as the normal force)

If the elevator is accelerating downwards, then, you can sort of imagine that the floor is 'TRYING' to move away from you. Which means less force on you? or less weight?
Does that give you some sort of intuition?

Answer 7

when you are going down, the net force is going down so aren't you suppose to be heavier

Answer 8

there is a greater force acting down

Answer 9

It is true that there is a net force downwards..
But that doesn't decide your weight.

Think another way.. when you move in a bus, if the bus accelerates forward, you feel an 'inertial push' backwards?
Similarly when the bus brakes, or when bus takes sudden left turn, you feel an inertial push forward, or rightwards?

Thus you always feel some kind of push in the opposite direction of the acceleration.

So when your elevator is accelerating downwards, you will "feel" an inertial push upwards, reduces your weight.
make sense?

Answer 10

but if there inertia is pushing you up, there must also be another force besides gravity pushing you down???

Answer 11

why? why would you say so?

Answer 12

and it is not 'inertia pushing you up'

it is, 'an inertial push'.. that is not really a force, it just FEELS that way

Answer 13

normal force is less than mg
there for mg( your weight) is heavier....

Answer 14

mg is NOT your weight.

mg = the gravitational force

your weight = Normal force..

now most textbooks say mg is your TRUE weight, and the normal force tells you your APPARENT weight.. but I don't like that definition..
I just say whatever is the normal force on you.. that is your weight!

Answer 15

mg is gravitational force which depends on your weight

Answer 16

normal force is force exerted by the elevator floor

Answer 17

Let me use some numbers

say your mass is 50kg, and g = 10
Then mg= 500N

say your elevator is accelerating down at 5 m/s^2
then Normal force is 250N.

(i am sure you can arrive at those numbers)

Now.. read carefully

Since the floor of the elevator is pushing you up with 250N of force, You my friend is also pushing DOWN on the floor with 250 N of force agree?
So if you put a scale underneath you.. that scale would register a 250N of force. and hence it would give you a reading of 25kg..

Answer 18

?

Answer 19

What part didn't you get?

Answer 20

why you are lighter if mg is greater

Answer 21

I just told you..
Your mg is always the same (on surface of the earth at least)

But that doesn't tell you much about your 'weight' or what you 'feel'

What you feel as weight is due to the fact that the ground is pushing you up.
you cannot just 'feel' forces.
This is actually quite subtle.. :P

Answer 22

so if you are feeling something then aren't you feeling more weight

Answer 23

That depends on which direction you are feeling.. :P

Answer 24

pushed down?

Answer 25

yea.. but in this case you are 'feeling a push up'

remember you always 'feel' in the opposite direction of the acceleration