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I understand that if a lift is accelerating downward (suppose at 2 $m/s^2$) then a man on the lift will feel a smaller normal reaction force so the resultant force is downward so the man can match the acceleration of the floor of the lift.

Also, if suppose a rope with tension $T$ was pulling up a block from the ground then also the normal reaction force falls. As $R=mg-T$ where $R$ is the normal reaction force.

Now, my question is how exactly does this happen at a microscopic level, like involving the repulsion of electrons between two surfaces. How does acceleration/movement of the surfaces cause the electron repulsion to change or some other way cause the normal reaction force to increase or decrease?

I believe it’s not a duplicate any question as they still does not answer how the repulsion works in an accelerated frame like for the floor of the lift accelerating downwards and the man’s feet.

Tausif Hossain
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2 Answers2

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How does acceleration/movement of the surfaces cause the electron repulsion to change or some other way cause the normal reaction force to increase or decrease?

There is nothing really special about it - it is all explainable by applying Newton's second law.

Take an example with an elevator. You are asking "How when the lift accelerates downwards there is no relative motion between the feet and the lift-floor, so how is there a change in force?"

The second law equation for a man in a lift looks like this:

$mg-N=ma$,

where $a$ is the acceleration of the lift.

From here we can find the normal reaction $N$ as:

$N=mg-ma$

So, $N$ depends on the lift acceleration: if the lift is accelerating down, $N$ will decrease, if the lift is accelerating up, $N$ will increase.

In other words, $N$ can change, while the contact between the floor of the lift and man's feet is maintained.

It is no different than pushing an object by hand with various degrees of force - you don't need to lose a contact with the object in order to reduce the force of the push. The distance between your hand and the object remains zero, but the molecules of your hand and the molecules of the object get less of a squeeze.

V.F.
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Now, my question is how exactly does this happen at a microscopic level, like involving the repulsion of electrons between two surfaces. How does acceleration/movement of the surfaces cause the electron repulsion to change or some other way cause the normal reaction force to increase or decrease?

Ah, the great question of physics: “But how does it know to do that?”

Think of the surfaces as being soft & squishy, a little springy. They might feel stiff, but at a small enough level, they have some give.

If the floor moves up, it’ll compress the surfaces & exert more force. Move a little more, a little more force. Eventually, the surface compresses enough to generate the right amount of force to get both bodies moving.

If the floor drops away, it’s similar: the force decreases as the spacing starts to increase. The supported body starts to fall. Too fast, it catches up, closes the gap, and more squishy force will start to slow it down.

Because the force changes with distance, more when closer and less with further, it forms an automatic feedback loop to hold the two bodies at (around) the same velocity.

For a visceral example, think of sitting in a big comfy chair: you sink in, with more and more pressure on your bottom, until there’s enough to hold you up. That’s done entirely automatically by the springs and cushions. A solid surface like a floor is just like that, except atomically small.

Bob Jacobsen
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