Our weight force (P = m.g), always pointing downwards, acts on a contact surface so that it applies on us a normal force (FN) always perpendicular, usually applied to our feet, and, in this case, pointing to up. Normal and weight, because they act on the same body, are not a pair of action and reaction.
When we enter an elevator, according to its movement we can feel different sensations. Remembering that according to Newton's 1st Law, the body, by inertia, tends to maintain its state, whether at rest or MRU. And according to the fundamental principle of Dynamics, the net force (FR) can be calculated by FR = m.a, where m is the mass of the body and a is the acceleration developed by it.
There are five possible cases:
- Elevator stopped or up and down at constant speeds (MRU):
In these cases, the normal force applied to our feet is equal to our weight force, as the only acceleration we are feeling is gravity. The resulting force between normal and weight is nil. FR = 0 --> FN = P
-Elevator starting its ascent movement:
To go up, the elevator makes an upward force, having a positive acceleration facing upwards. As the resultant is up, the normal force is greater than the weight force. FN > P --> FR = FN - P
-Elevator finishing its ascent movement:
To stop, the elevator decelerates making the resultant face down, causing it to brake. P > FN --> FR = P - FN
-Elevator starting the descent movement:
Since it is descending at an accelerated rate, its resultant is facing downwards. P > FN --> FR = P - FN
-Elevator ending the descent movement:
As the elevator is descending, apply an upward force to stop. FN > P --> FR = FN - P
As our force P is always constant, a scale placed on the elevator floor would indicate the value of the force FN applied in each case. As an example, a person of mass = 60 Kg, in an elevator that goes up and down with an acceleration of 3 m/s², it has a force P = 600 N (considering g = 10 m/s²) and the scale will indicate its FN ranging from 420 N to 780 N. With this we can disprove the false idea that the scale measures our weight force, it measures the normal force of reaction to weight, which, as we have seen, can vary depending on the situation.
A curiosity is that if the elevator descends with an acceleration equal to gravity (it simply falls under the gravity), its normal force is nil (FN = 0), so the person would float within the elevator. This same effect is used in free-fall planes for training pilots and astronauts, simulating weightlessness.
by Luciano Calaça
Graduated in Physics
Brazil school team
See more: Strength
mechanics - Physics - Brazil School
Source: Brazil School - https://brasilescola.uol.com.br/fisica/a-fisica-no-elevador.htm