# Man on an Elevator -- Force Diagrams

• Lugytopo
In summary, the conversation discussed the confusion over velocity and acceleration in relation to force diagrams and scale readings. It was mentioned that the velocity only indicates direction and does not affect the force diagrams or scale readings. The equation "sum of forces equals mass times resulting acceleration" was also mentioned as a key concept to understand. The conversation also touched on the use of Newtons instead of kilograms when discussing forces, and the need to write the relevant equation for each case.
Chestermiller said:
The force balance on the man is $$N-mg=ma$$where N is the upward force that the scale exerts on the man, mg is the downward force of the Earth on the man, and a is the upward acceleration. Does that make sense?
Well, let's see. We can change that equation to N= ma + mg.
ma =(61.2*11.8)= 722.2
mg= (61.2*9.8)= 599.8

So N=1322 N--am I understanding this correctly?

Lnewqban said:
The thing is that, when the elevator is not moving, the man is being accelerated upwards by the scale at exactly the same rate as the gravity is trying to accelerate him downwards (like in a free-fall), resulting in no net acceleration respect to Earth in any direction.
Action force of feet against scale equals reaction force of scale against feet and there is no change of state (repose or constant up or down velocity).

That balance is lost once the elevator starts pushing the scale, and the scale pushes the man upwards with a force greater than his normal weight (when in repose).
This seems very confusing to me, especially the part about the man being accelerated upwards by the scale when the elevator is not moving.

Or would the force just be 600 + (61.2 * 2)= 722.4?

Lugytopo said:
Or would the force just be 600 + (61.2 * 2)= 722.4?
That is correct

Chestermiller said:
That is correct
Alright, thanks for the help.

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