1. The elements
A, B, X, Y and Z have atoms with outer electron shell configurations containing
1, 2, 4, 7 and 8 electrons respectively. State and describe the type of bonding
which is likely to occur in the following cases:
The angle of repose or, more precisely, the critical angle of repose,[1] of a granular material is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is in the range 0°–90°.
When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose.
The term has a related usage in mechanics, where it refers to the maximum angle at which an object can rest on an inclined plane without sliding down. This angle is equal to the arctangent of the coefficient of static friction μs between the surfaces.
An
object placed on a tilted surface will often slide
down the surface. The rate at which the object slides down
the surface is dependent upon how tilted the surface
is; the greater the tilt of the surface, the faster
the rate at which the object will slide down it. In physics,
a tilted surface is called an inclined plane. Objects
are known to accelerate down inclined planes because of an
unbalanced force. To understand this type of motion, it is
important to analyze the forces acting upon an object on an
inclined plane. The diagram at the right depicts the two
forces acting upon a crate that is positioned on an
inclined plane (assumed to be friction-free). As shown in
the diagram, there are always at least two forces
acting upon any object that is positioned on an inclined
plane - the force of gravity and the normal force. The
force of gravity (also
known as weight) acts in a downward direction; yet the
normal force acts in a
direction perpendicular to the surface (in fact,
normal means "perpendicular"). The first peculiarity of inclined plane
problems is that the normal force is not directed in
the direction that we are accustomed to. Up to this point
in the course, we have always seen normal forces acting in
an upward direction, opposite the direction of the force of
gravity. But this is only because the objects were always on
horizontal surfaces and never upon inclined planes. The
truth about normal forces is not that they are always
upwards, but rather that they are always directed
perpendicular to the surface that the object is on.
The task of determining the net force
acting upon an object on an inclined plane is a difficult
manner since the two (or more) forces are not directed in
opposite directions. Thus, one (or more) of the forces will
have to be resolved into perpendicular components so that
they can be easily added to the other forces acting upon the
object. Usually, any force directed at
an angle to the horizontal is resolved into horizontal and
vertical components. However, this is not the process
that we will pursue with inclined planes. Instead, the
process of analyzing the forces acting upon objects on
inclined planes will involve resolving the weight vector
(Fgrav) into two perpendicular components. This
is the second peculiarity of inclined plane problems. The
force of gravity will be resolved into two components of
force - one directed parallel to the inclined surface and
the other directed perpendicular to the inclined surface.
The diagram below shows how the force of gravity has been
replaced by two components - a parallel and a perpendicular
component of force.
The perpendicular
component of the force of gravity is directed opposite the
normal force and as such balances the normal force. The
parallel component of the force of gravity is not balanced
by any other force. This object will subsequently accelerate
down the inclined plane due to the presence of an unbalanced
force. It is the parallel component of the force of gravity
that causes this acceleration. The parallel component of
the force of gravity is the net force.
The task of determining the magnitude of
the two components of the force of gravity is a mere manner
of using the equations. The equations for the parallel and
perpendicular components are:
In the absence of friction and other forces (tension,
applied, etc.), the acceleration of an object on an incline
is the value of the parallel component (m*g*sine of angle)
divided by the mass (m). This yields the equation
(in the absence of friction and
other forces)
In
the presence of friction or other forces (applied force,
tensional forces, etc.), the situation is slightly more
complicated. Consider the diagram shown at the right. The
perpendicular component of force still balances the normal
force since objects do not accelerate perpendicular to the
incline. Yet the frictional force must also be considered
when determining the net force. As in all net force
problems, the net force
is the vector sum of all the forces. That is, all the
individual forces are added together as vectors. The
perpendicular component and the normal force add to 0 N. The
parallel component and the friction force add together to
yield 5 N. The net force is 5 N, directed along the incline
towards the floor. The above problem (and all inclined plane
problems) can be simplified through a useful trick known as
"tilting the head." An inclined plane problem is in every
way like any other net force problem with the sole exception
that the surface has been tilted. Thus, to transform
the problem back into the form with which you are more
comfortable, merely tilt your head in the same
direction that the incline was tilted. Or better yet,
merely tilt the page of paper (a sure remedy for TNS
- "tilted neck syndrome" or "taco neck syndrome") so that
the surface no longer appears level. This is illustrated
below.
Once the force of gravity has been resolved into its two
components and the inclined plane has been tilted, the
problem should look very familiar. Merely ignore the force
of gravity (since it has been replaced by its two
components) and solve for the net force and
acceleration. As
an example consider the situation depicted in the diagram at
the right. The free-body diagram shows the forces acting
upon a 100-kg crate that is sliding down an inclined plane.
The plane is inclined at an angle of 30 degrees. The
coefficient of friction between the crate and the incline is
0.3. Determine the net force and acceleration of the
crate. Begin the above problem by finding the
force of gravity acting upon the crate and the components of
this force parallel and perpendicular to the incline. The
force of gravity is 980 N and the components of this force
are Fparallel = 490 N (980 N • sin 30
degrees) and Fperpendicular = 849 N (980 N •
cos30 degrees). Now the normal force can be determined to be
849 N (it must balance the perpendicular
component of the weight vector). The force of friction
can be determined from the value of the normal force and the
coefficient of friction; Ffrict is 255 N
(Ffrict = "mu"*Fnorm= 0.3 • 849
N). The net force is the vector sum of all the forces. The
forces directed perpendicular to the incline balance; the
forces directed parallel to the incline do not balance. The
net force is 235 N (490 N - 255 N). The acceleration is 2.35
m/s/s (Fnet/m = 235 N/100 kg).
Practice
The two diagrams below depict the free-body diagram for a
1000-kg roller coaster on the first drop of two different
roller coaster rides. Use the above principles of vector
resolution to determine the net force and acceleration of
the roller coaster cars. Assume a negligible affect of
friction and air resistance. When done, click the button to
view the answers.
The affects of the incline angle on the
acceleration of a roller coaster (or any object on an
incline) can be observed in the two practice problems above.
As the angle is increased, the acceleration of the object is
increased. The explanation of this relates to the components
that we have been drawing. As the angle increases, the
component of force parallel to the incline increases and the
component of force perpendicular to the incline decreases.
It is the parallel component of the weight vector that
causes the acceleration. Thus, accelerations are greater at
greater angles of incline. The diagram below depicts this
relationship for three different angles of increasing
magnitude.
Roller
coasters produce two thrills associated with the initial
drop down a steep incline. The thrill of acceleration is
produced by using large angles of incline on the first drop;
such large angles increase the value of the parallel
component of the weight vector (the component that causes
acceleration). The thrill of weightlessness is
produced by reducing the magnitude of the normal force to
values less than their usual values. It is important to
recognize that the thrill of weightlessness is a feeling
associated with a lower than usual normal force. Typically,
a person weighing 700 N will experience a 700 N normal force
when sitting in a chair. However, if the chair is
accelerating down a 60-degrees incline, then the person will
experience a 350 Newton normal force. This value is less
than normal and contributes to the feeling of weighing less
than one's normal weight - i.e.,
weightlessness. Angles of Friction Equations
tan φs = μs
tan φk = μk
Nomenclature
symbol
description
φs
angle of static friction
φk
angle of kinetic friction
μs
coefficient of static friction
μk
coefficient of kinetic friction
N
normal component of the reaction surface
Explanation
It is sometimes convenient to replace the normal forceN and the friction force F by a resultant R. Consider a block of weight W resting on a horizontal plane surface:
No friction
R = N
If no horizontal force is applied to the block, the resultant R reduces to the normal force N.
No motion
F = Px φ < φs
However, if the applied forceP has a horizontal component Px which tends to move the block, the force R will have a horizonatal component F and, thus, will form an angle φ with the normal to the surface as shown.
Motion impending
Fm = Px φ = φs
If Px is increased until motion becomes impending, the angle between R and the vertical grows and reaches a maximum value.
This maximum value is called the angle of static friction and is denoted by φs. From the geometry of the previous figure, the equation is:
tan φs =
Fm
N
=
μsN
N
tan φs = μs
Motion
F = Px φ < φs
If motion actually takes place, the magnitudes of the friction force drops to Fk; similarly, the angle φ between R and N drops to a lower value φk, called the angle of kinetic friction.
From the geometry of the previous figure, the equation is:
tan φk =
Fk
N
=
μkN
N
tan φk = μk
Another example may show how the angle of friction can be used to
advantage in the analysis of certain types of problems. Consider a block
resting on a base and subjected to no other force than its weight W and the reaction R of the board. The board can be given any desired inclination.
No friction
R = N
If the base is horizontal, the force R exerted by the board on the block is perpendicular to the base and balances the weight W.
No motion
F = Px φ < φs
If the base is given a small angle of inclination, θ, the force R will deviate from the perpendicular to the base by the angle θ and will keep balancing W; it will then have a normal componenet N of magnitude N = W cos θ and a tangential component F of magnitude F = W sin θ.
Motion impending
Fm = Px φ = φs
If the angle of inclination continues to increase, motion will soon become impending. At that time, the angle between R and the normal will have reached its maximum value φs.
The value of the angle of inclination corresponding to impending motion
is called the angle of repose. Clearly, the angle of repose is equal to
the angle of static friction φ.
Motion impending
Fm = Px φ = φs
If the angle of inclination θ is further increased, motion starts and the angle between R and the normal drops to the lower value φk. The reaction R is not vertical any more, and the forces acting on the block are unbalanced.
Friction is both good and bad: it allows objects to move, such as in walking or driving, yet can result in a lot of wasted energy through the generation of heat, such as in engines, and in other areas.
Friction can be understood simply by observing the following picture of a cinder block sitting on asphalt:
Zooming in, it can be seen that there are irregularities in the
surfaces of both materials, and when they are moved against each other,
these small bits are pushing against each other. It is the summation of
all of these horizontal forces
causing resistance that make it hard to push an object with a lot of
friction. When an object is pushed the bits may break off, or they may
bump over each other. The weight
of the object, or any vertical forces that are pushing down on the
object, will influence the friction because as vertical downward force
is increased, it becomes harder to overcome those horizontal forces
between all the little small bits interconnected with each other between
the contacting surfaces.
The laws of dry friction may be explained with a wooden block of weight W that is placed on a horizontal concrete plane surface as shown below.
The forces acting on the block are its weight W
and the reaction of the surface. Since the weight has no horizontal
component, the reaction of the surface also has no horizontal component.
The reaction is therefore normal to the surface and is represented by N as shown above.
Now a horizontal force P is applied to the block as shown in the following figure:
If P is small enough, the block will not move. This only
means that some other horizontal force must therefore exist, which
balances P. This other force is the static-friction force F,
which is actually the resultant of numerous forces acting over the
entire surface of contact between the block and the plane, as indicated
in the first picture in this lesson. It is generally assumed that these
forces are due to the irregularities of the surfaces in contact and, to a
certain extent, to molecular attraction.
If the force P is increased, the friction force F also increases, continuing to oppose P until its magnitude reaches a certain maximum value Fm, as shown in the following plot:
If P is further increased, the friction force cannot balance
it any more and the block starts sliding. A more accurate representation
of the previous plot may be the following:
F and P are relatively proportional until Fm is reached, and motion begins. Fk
may be shown as a wavy and bumpy line because the frictional force
varies at the micro-level interactions, as seen in the first picture in
this lesson, which may not be consistent.
Notice in the following figure that, as the magnitude F of the friction force increases from 0 to Fm, the point of application a of the resultant N of the normal forces of contact moves to the right, so that the couples formed, respectively, by P and F and by W and N remain balanced.
If N reaches point b, which is fixed at the bottom corner of the block, before F reaches its maximum value Fm, the couples will no longer be balanced and the block will tip about point b before the block can start sliding.
As soon as the block has been set in motion, the magnitude of F drops from Fm to a lower value Fk as shown below and in the above plots:
This is because there is less interpenetration between the
irregularities of the surfaces in contact when these surfaces move with
respect to each other. From then on, the block keeps sliding with
increasing velocity while the friction force is now denoted by Fk,
which is called the kinetic friction force. This can be observed in the
plot above. The kinetic friction force remains approximately constant
throughout the motion of the block.
Experimental evidence shows that the maximum value Fm of the static friction force is proportional to the normal component N of the reaction of the surface. The equation is:
(Eq1)
Fm = μsN
Where μs is a constant called the coefficient of static friction. Similarly, the magnitude Fk of the kinetic friction force may be put in the form:
(Eq2)
Fk = μkN
Where μk is a constant called the coefficient of kinetic friction. The coefficients of friction μs and μk
do not depend upon the area of the surfaces in contact. Both
coefficients, however, depend strongly on the nature of the surfaces in
contact. Since they also depend upon the exact condition of the
surfaces, their value is seldom known with an accuracy greater than 5
percent. The corresponding values of the coefficient of kinetic friction
would be about 25 percent smaller.
It is presumed that four different situations can occur when a rigid body is in contact with a horizontal surface:
1.
No friction (Px = 0)
F = 0 N = P + W
The forces applied to the body do not tend to move it along the surface of contact; there is not friction force.
2.
No motion (Px < Fm)
F = Px F < μsN N = Py + W
The
applied forces tend to move the body along the surface of contact but
are not large enough to set it in motion. The friction force F which has developed can be found by solving the equations of equilibrium for the body. Since there is no evidence that F has reached its maximum value, the equation Fm = μsN cannot be used to determine the friction force.
3.
Motion impending (Px = Fm)
Fm = Px Fm = μsN N = Py + W
The applied forces are such that the body is just about to slide, that is, motion is impending. The friction force F has reached its maximum value Fm and, together with the normal force N, balances the applied forces. Both the equations of equilibrium and the equation Fm = μsN can be used. Also note that the friction force has a sense opposite to the sense of impending motion.
4.
Motion (Px > Fm)
Fk < Px Fk = μkN N = Py + W
The body is sliding under the action of the applied forces, and the equations of equilibrium do not apply any more. However, F is now equal to Fk and the equation Fk = μkN may be used. The sense of Fk is opposite to the sense of motion.
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