If normal force is self adjusting,why does a thin wooden plank break when a very heavy body is placed on it?

Question

If normal force is self adjusting,why does a thin wooden plank break when a very heavy body is placed on it?

Answer 1

The breakage depends on the roughness of the surface.

If the particles of the wooden plank find any place to move to because of the large force exerted by the heavy body, they would move there. Whereas the rest which don't find any place will not. Thus, there won't be any uniformity in the effect of force on the surface and the wooden plank would bread.

Answer 2

The normal force is provided by the material/surface that the object is on, and this force has a limit, depending on the that material/surface.  

In this case.. the thin wooden plank.  When a heavy body is placed on the wooden plank, it's weight pushes on the plank.  Now the plank tries to 'provide' a normal force on the heavy body to hold it up.  But if the weight of the body is higher than the maximum normal force that the plank can provide, then the plank breaks.. and gives in to the body's weight.

The normal force is only 'self-adjusting' up to a certain limit.  It always self-adjusts to be equal to the force applied to that surface if the force is less than its limit.

Answer 3

By Newton's Third law

"Every action has an equal and opposite reaction."

Assuming a horizontal plank, for example, the weight of the body is the normal force. The plank provides a reaction force to the body's weight "force".

but:

Once the plank's yield point is reached the material

begins to deform plastically under the stress provided by

the weight. At some point, the plank will break as more

weight is added

Answer 4

"normal force is self adjusting", this has no sense. You must quote it in context.