# 3rd Law Question

## Main Question or Discussion Point

Ok I always understood the 3rd law and normal force well until recently when I started overthinking it. In an elevator, your weight pushes down on say a scale. That scale pushes back onto you with an equal and opposite force. But if the elevator is accelerating upwards, then do you exert an even greater force on the scale and the scale in turn exerts an even greater force back on you? How does the 3rd law apply in an accelerating elevator?

## Answers and Replies

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DrClaude
Mentor
But if the elevator is accelerating upwards, then do you exert an even greater force on the scale and the scale in turn exerts an even greater force back on you?
Yes, the same way that you press harder against the seat in an accelerating vehicle.

A.T.
Science Advisor
In an elevator, your weight pushes down on say a scale.
No, "your weight" pulls down on you. The 3rd Law opposite force to this is you pulling on the Earth.

That scale pushes back onto you with an equal and opposite force.
That is another 3rd Law pair. Both are contact forces. None of them is weight. And they don't have to be equal to your weight.

So then how is there a net force to force your body to accelerate in that situation?

jbriggs444
Science Advisor
Homework Helper
2019 Award
So then how is there a net force to force your body to accelerate in that situation?
The force of gravity on your body is not equal to the force of the elevator on your body. The net is non-zero.

The force of gravity on your body is not equal to the force of the elevator on your body. The net is non-zero.
Ok, so there is a net force upward because your weight is not as large as the the force of the scale on you. The force you exert on the scale as part of the 3rd law doesn't really matter right?

jbriggs444
Science Advisor
Homework Helper
2019 Award
Ok, so there is a net force upward because your weight is not as large as the the force of the scale on you. The force you exert on the scale as part of the 3rd law doesn't really matter right?
Right!

The force you exert on the scale counts toward the net force on the scale, not toward the net force on you.

sophiecentaur
Science Advisor
Gold Member
So then how is there a net force to force your body to accelerate in that situation?
Do not confuse the Third Law with notions of equilirium and the First Law. The third law is to do with the interaction between two objects and the conservation of Momentum. The First Law is to do with what happens to an object when subjected to one of more forces.

I see. Like I said I was overthinking, and the last thing I need to clear up is if you exert a force on that scale, what is stopping that scale from accelerating downwards? The force of the elevator floor? And the elevator is accelerating most likely through tension so that's why the scale doesn't move downwards right?

sophiecentaur
Science Advisor
Gold Member
I see. Like I said I was overthinking, and the last thing I need to clear up is if you exert a force on that scale, what is stopping that scale from accelerating downwards? The force of the elevator floor? And the elevator is accelerating most likely through tension so that's why the scale doesn't move downwards right?
Any downward force on the floor could affect the speed of ascent or descent but the whole machine is very inefficient (lots of friction) and the effect could be undetectable.

Alright thank you all!