B1 中級 美國腔 55 分類 收藏
In this episode, let's talk about the relationship between the force and objects ,
first this blue is an object,
it has contains both are convex and concave,
and also relatively wider and narrower parts,
when after the concept of the objects enters, we will to produce the proportional scale,
the unit is what that not the focus,
this an object is subjected to variety of different roles by the forces and moment couple now,
so it produced a certain the deformation,
and the dashed line is the object line after it's deformed,
maybe it still will start to moves or rotations after thrust surface,
the force will causes objects the moves dip direction,
the moment couple will causes objects to have rotates dip direction,
the soil mechanics did not use to the moment couple,
so it will not appear again after this episode appears.
that, we consider of the objects are also not rotation occurs ,
because the soil will not rotation,
ok, next let's take a look at the force.
first, we can see it withstand the concentrated force of F1
and the force of F2
there are also f1 and f2
the uniform force and distributed force,
that, let's explain the nouns now,
focusing on role by the object on 1 point is called the concentrated force,
the concentrated force can act on or outside the objects,
can be understood as the force at a distance, (but actually is vector movement), that's no problem,
the force of unit we use is Newton N,
the concentrated force actually is abstract concept,
assume the force acts on a certain particle,
this particle doesn't exist in the real world,
the force of real world must be demonstrated in the distributed force manner,
even if you poke the potato with one needle,
the tip also has certain area,
next, let's look at the distributed force,
the distributed force can be called uniformity force in certain case,
that is, it the same with role of "density"form on a certain acreage,
that used here was is similar to the concept of density,
the distributed force unit actually is how many Newtons is acts on per square meter,
actually also is mean the how many Newtons is acts on each unit area,
in fact, the distribution force can be equivalent to the concentrated force,
the equivalent manner is as follows:
first to consider this the uniform force ,
the density of the uniform force is f2
the interaction length is l2,
what is the equivalent concentrated force of such a force?
write the concentrated force as the red arrow,
it's value = f2 * l2
then multiply the unit area is 1
this 1 can be as length of paper surface prominence,
we use angle of the three dimension to see is so
so it acts on an area,
the magnitude by the concentrated force is the volume of the rectangular parallelopiped,
i.e f2*l2*1
where is such a the concentrated force acting?
acting on above the centroid of the rectangle,
the centroid is in the middle...so we draw the concentrated force right here,
point of application in about 1/2l2 the place,
another one distributed force is the triangle,
in fact, the distributed force can have any shape,
the triangle...that accordance with the rectangle,
the magnitude is the bottom multiplied by the height divided by two
point of application in 1/3l1 the place,
when in the exam, we used to take the uniform force
to calculates after change to the concentrated force,
because for the objects ,them are equivalent ,
the so-called the equivalent is the same with the effect of generated,
that let's consider a question now.
what happens after an object is cuts ?
we know the object is for the maintain its shape before the thrust surface,
when subjected to external forces, internal forces are also will generated within the object,
so soil mechanics is defined...
the force that can be observed before cutting is the external force
the force that emerges after being cut is called the internal force.
the internal force is actually very complicated,
and with the angle or way you cut,
the internal forces generated both are also different,
the internal forces must have appeard with form of distributed forces,
it main can be divided into two types,
one is the normal stress and one is the tangential stress,
the so-called the normal stress is the direction of stress vertical to the section,
the so-called the tangential stress is the direction of stress parallel to the section,
such as red arrows
the external force and internal force are gives to objects...
effects produced are not the same,
what's the effects produced of the external force?
has movement, rotation, reaction force, work and so on ...
for the internal force, it will make the...
the stress in the object changes,
deformation, transformation energy...and so on.
for in soil mechanics...
when the external force continue to increase in the case, the internal force will rise accordingly,
the normal stress in the soils will causes soil consolidation,
that is the density increases (strength rises),
the increase of the tangential stress will make the the general shear failure of soils,
the main form of soil damage is the general shear failure,
finally, let's supplement the concept of "the rigid body",
the rigid body is considered as the ideal objects for the internal effects are not exist.
can imagine the rigid body don't exist in the real world,
within the subjects of engineering mechanics is
the objects are all assumed be the rigid bodies,,
that is, only the effects of external and external effects are considered,
the material mechanics will considered of internal force the part,
then the soil mechanics is builted on top of these two subjects,
so the scope of consideration is will again broader,
ok, the above is all the content today. Bye ~



55 分類 收藏
WC 發佈於 2020 年 7 月 7 日
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