Page 1
Edurev123
STATICS
1. Fundamental ideas,
Statics deals with forces (on bodies) which produce equilibrium.
If a single force ?? acting indepencently produces the same effect as that due to a
number of forces ?? 1
,?? 2
,?? 3
, etc., acting simultaneously; then ???
is called the resustant of
???
1
,???
2
,???
3
, etc. in other words ????
is ???
1
+???
2
+???
3
+?, The forces ???
1
,???
2
,???
3
, etc., are called
components of ????
.
The resultant of two forces :??? and ???. along ???? and ???? such that angle ?? ?? ?? is ?? . is of
magnitude v?? 2
+?? 2
+2???? cos ?? . This is obtained by applying parallelogram law for
addition of vectors.
If two forces are in equilibrium, then they must be of equal magnitude and act in opposite
directions along the same line.
If three forces acting at a point are in equilibrium, they can be represented in magnitude
and direction by the sides of a triangle taken in order. This is known as law of triangle of
forces.
The converse of this law is also true.
From the law of triangle of forces we get Lami's, theorem. If three forces acting at a
point, are in equilibrium, then each force is proportional to the sine of the angle between
the other two and conversely.
The magnitude of the resultant ?? of two like parallel forces ????
and ????
acting at ?? and ?? is
sum of their magnitudes, that is ?? i. ?? , and
the line of action of ????
(which is parallel to that of ?? and ?? ) divides ???? internally inversely
in the ratio of the forces ?? and ?? .
Page 2
Edurev123
STATICS
1. Fundamental ideas,
Statics deals with forces (on bodies) which produce equilibrium.
If a single force ?? acting indepencently produces the same effect as that due to a
number of forces ?? 1
,?? 2
,?? 3
, etc., acting simultaneously; then ???
is called the resustant of
???
1
,???
2
,???
3
, etc. in other words ????
is ???
1
+???
2
+???
3
+?, The forces ???
1
,???
2
,???
3
, etc., are called
components of ????
.
The resultant of two forces :??? and ???. along ???? and ???? such that angle ?? ?? ?? is ?? . is of
magnitude v?? 2
+?? 2
+2???? cos ?? . This is obtained by applying parallelogram law for
addition of vectors.
If two forces are in equilibrium, then they must be of equal magnitude and act in opposite
directions along the same line.
If three forces acting at a point are in equilibrium, they can be represented in magnitude
and direction by the sides of a triangle taken in order. This is known as law of triangle of
forces.
The converse of this law is also true.
From the law of triangle of forces we get Lami's, theorem. If three forces acting at a
point, are in equilibrium, then each force is proportional to the sine of the angle between
the other two and conversely.
The magnitude of the resultant ?? of two like parallel forces ????
and ????
acting at ?? and ?? is
sum of their magnitudes, that is ?? i. ?? , and
the line of action of ????
(which is parallel to that of ?? and ?? ) divides ???? internally inversely
in the ratio of the forces ?? and ?? .
If two unlike parallel forces ????
and ????
act at ?? and ?? , the magnitude of their "resultant ????
is
?? Q and the tine of action of ????
divides ???? externally inversely in the ratio of the forces.
The moment about a point ?? of a force ???
whose line of action passes through a point ??
is ????
¯¯¯¯
×???
=???×???
If a number of forces act at a point, the algebraic sum of their (rectangular) components
along any direction is equal to the component of their resultant in the same direction; the
vector sum of their moments about any point is equal to the moment of their resultant
about the same point.
It follows that if a number of forces acting at a point are in equilibrium the algebraic sum
of their components along each of three mutually perpendicular directions must be zero
or the vector sum of their components about each of three non-collinear points must be
zero. Fic
If three forces are in equilibrium, then they must be co-planar and they must be either
concurrent or all of them must be parallel.
The resultant of two forces acting at a point O along ???? and ???? and represented in
magnitude by ?? : ?? A and ?? ,?? 3 is represented by (?? +?? )???? where ?? is a point on ????
such that ?? CA=?? ?? CB
A useful trigonometrical result
?? is a point on the side ?? B of a triangle ?????? such that ???? :???? =?? :??
Then
(?? +?? )cot?? =?? cot a-?? cot ??
(?? +?? )cot ?? =?? cos ?? -?? cot ??
Page 3
Edurev123
STATICS
1. Fundamental ideas,
Statics deals with forces (on bodies) which produce equilibrium.
If a single force ?? acting indepencently produces the same effect as that due to a
number of forces ?? 1
,?? 2
,?? 3
, etc., acting simultaneously; then ???
is called the resustant of
???
1
,???
2
,???
3
, etc. in other words ????
is ???
1
+???
2
+???
3
+?, The forces ???
1
,???
2
,???
3
, etc., are called
components of ????
.
The resultant of two forces :??? and ???. along ???? and ???? such that angle ?? ?? ?? is ?? . is of
magnitude v?? 2
+?? 2
+2???? cos ?? . This is obtained by applying parallelogram law for
addition of vectors.
If two forces are in equilibrium, then they must be of equal magnitude and act in opposite
directions along the same line.
If three forces acting at a point are in equilibrium, they can be represented in magnitude
and direction by the sides of a triangle taken in order. This is known as law of triangle of
forces.
The converse of this law is also true.
From the law of triangle of forces we get Lami's, theorem. If three forces acting at a
point, are in equilibrium, then each force is proportional to the sine of the angle between
the other two and conversely.
The magnitude of the resultant ?? of two like parallel forces ????
and ????
acting at ?? and ?? is
sum of their magnitudes, that is ?? i. ?? , and
the line of action of ????
(which is parallel to that of ?? and ?? ) divides ???? internally inversely
in the ratio of the forces ?? and ?? .
If two unlike parallel forces ????
and ????
act at ?? and ?? , the magnitude of their "resultant ????
is
?? Q and the tine of action of ????
divides ???? externally inversely in the ratio of the forces.
The moment about a point ?? of a force ???
whose line of action passes through a point ??
is ????
¯¯¯¯
×???
=???×???
If a number of forces act at a point, the algebraic sum of their (rectangular) components
along any direction is equal to the component of their resultant in the same direction; the
vector sum of their moments about any point is equal to the moment of their resultant
about the same point.
It follows that if a number of forces acting at a point are in equilibrium the algebraic sum
of their components along each of three mutually perpendicular directions must be zero
or the vector sum of their components about each of three non-collinear points must be
zero. Fic
If three forces are in equilibrium, then they must be co-planar and they must be either
concurrent or all of them must be parallel.
The resultant of two forces acting at a point O along ???? and ???? and represented in
magnitude by ?? : ?? A and ?? ,?? 3 is represented by (?? +?? )???? where ?? is a point on ????
such that ?? CA=?? ?? CB
A useful trigonometrical result
?? is a point on the side ?? B of a triangle ?????? such that ???? :???? =?? :??
Then
(?? +?? )cot?? =?? cot a-?? cot ??
(?? +?? )cot ?? =?? cos ?? -?? cot ??
Example 1
Forces acting in at a point I inside a quadrilateral ???????? represented by ???? ,???? ,???? and
?? D in magnitude and direction are in equilibrium. Find the position of ?? .
Let ?? and ?? be the midpoint of ???? and ???? respectively.
Then
PB
¯¯¯¯
·PC
¯¯¯¯
=2PN
¯¯¯¯
PD
¯¯¯¯
·PA
¯¯¯¯
=2PM
¯¯¯¯
We get
2(???
|???
+????
¯¯¯¯
)=????
¯¯¯¯
+???
+????
¯¯¯¯
+????
¯¯¯¯
=0 (given)
Therefore ?? is the midpoint of ?? ?? .
It is also the midpoint of the line joining the midpoints of ???? and ???? .
Example 2
A uniform circular plate of weight ?? is supported at three points in its edge whose
distances apart are a, b, c. Find the load carried by each support.
Page 4
Edurev123
STATICS
1. Fundamental ideas,
Statics deals with forces (on bodies) which produce equilibrium.
If a single force ?? acting indepencently produces the same effect as that due to a
number of forces ?? 1
,?? 2
,?? 3
, etc., acting simultaneously; then ???
is called the resustant of
???
1
,???
2
,???
3
, etc. in other words ????
is ???
1
+???
2
+???
3
+?, The forces ???
1
,???
2
,???
3
, etc., are called
components of ????
.
The resultant of two forces :??? and ???. along ???? and ???? such that angle ?? ?? ?? is ?? . is of
magnitude v?? 2
+?? 2
+2???? cos ?? . This is obtained by applying parallelogram law for
addition of vectors.
If two forces are in equilibrium, then they must be of equal magnitude and act in opposite
directions along the same line.
If three forces acting at a point are in equilibrium, they can be represented in magnitude
and direction by the sides of a triangle taken in order. This is known as law of triangle of
forces.
The converse of this law is also true.
From the law of triangle of forces we get Lami's, theorem. If three forces acting at a
point, are in equilibrium, then each force is proportional to the sine of the angle between
the other two and conversely.
The magnitude of the resultant ?? of two like parallel forces ????
and ????
acting at ?? and ?? is
sum of their magnitudes, that is ?? i. ?? , and
the line of action of ????
(which is parallel to that of ?? and ?? ) divides ???? internally inversely
in the ratio of the forces ?? and ?? .
If two unlike parallel forces ????
and ????
act at ?? and ?? , the magnitude of their "resultant ????
is
?? Q and the tine of action of ????
divides ???? externally inversely in the ratio of the forces.
The moment about a point ?? of a force ???
whose line of action passes through a point ??
is ????
¯¯¯¯
×???
=???×???
If a number of forces act at a point, the algebraic sum of their (rectangular) components
along any direction is equal to the component of their resultant in the same direction; the
vector sum of their moments about any point is equal to the moment of their resultant
about the same point.
It follows that if a number of forces acting at a point are in equilibrium the algebraic sum
of their components along each of three mutually perpendicular directions must be zero
or the vector sum of their components about each of three non-collinear points must be
zero. Fic
If three forces are in equilibrium, then they must be co-planar and they must be either
concurrent or all of them must be parallel.
The resultant of two forces acting at a point O along ???? and ???? and represented in
magnitude by ?? : ?? A and ?? ,?? 3 is represented by (?? +?? )???? where ?? is a point on ????
such that ?? CA=?? ?? CB
A useful trigonometrical result
?? is a point on the side ?? B of a triangle ?????? such that ???? :???? =?? :??
Then
(?? +?? )cot?? =?? cot a-?? cot ??
(?? +?? )cot ?? =?? cos ?? -?? cot ??
Example 1
Forces acting in at a point I inside a quadrilateral ???????? represented by ???? ,???? ,???? and
?? D in magnitude and direction are in equilibrium. Find the position of ?? .
Let ?? and ?? be the midpoint of ???? and ???? respectively.
Then
PB
¯¯¯¯
·PC
¯¯¯¯
=2PN
¯¯¯¯
PD
¯¯¯¯
·PA
¯¯¯¯
=2PM
¯¯¯¯
We get
2(???
|???
+????
¯¯¯¯
)=????
¯¯¯¯
+???
+????
¯¯¯¯
+????
¯¯¯¯
=0 (given)
Therefore ?? is the midpoint of ?? ?? .
It is also the midpoint of the line joining the midpoints of ???? and ???? .
Example 2
A uniform circular plate of weight ?? is supported at three points in its edge whose
distances apart are a, b, c. Find the load carried by each support.
S (cicumcentre of, BC ) is the centre of the circuar plate. The weight ?? acts at ?? vertically
downward. Let ?? 1
,?? 2
,?? 3
be the forces acting vertically upward on the plate at ?? ,?? , C
respectively.
The forces acting on the plate are in equilibrium. Taking moments of the forces about the
line [???? , we get ?? 1
???? =?? ·?? ?? '
??? 1
:?? ·
?? ?? '
????
=?? ·
2cos ?? 2sin ?? .(?? is circum radius =SA )
(?
?? sin ?? =2?? and ?? 2
=?? 2
+?? 2
-2???? ·cos ?? )
=?? ?? 2
2cos ?? ????
=?? ?? 2
?? 2
?? 2
(?? 2
+?? 2
-?? 2
)
=
?? ?? 2
?? 2
?? 2
?? 2
[?? 2
(?? 2
+?? 2
-?? 2
)]
Therefore
Page 5
Edurev123
STATICS
1. Fundamental ideas,
Statics deals with forces (on bodies) which produce equilibrium.
If a single force ?? acting indepencently produces the same effect as that due to a
number of forces ?? 1
,?? 2
,?? 3
, etc., acting simultaneously; then ???
is called the resustant of
???
1
,???
2
,???
3
, etc. in other words ????
is ???
1
+???
2
+???
3
+?, The forces ???
1
,???
2
,???
3
, etc., are called
components of ????
.
The resultant of two forces :??? and ???. along ???? and ???? such that angle ?? ?? ?? is ?? . is of
magnitude v?? 2
+?? 2
+2???? cos ?? . This is obtained by applying parallelogram law for
addition of vectors.
If two forces are in equilibrium, then they must be of equal magnitude and act in opposite
directions along the same line.
If three forces acting at a point are in equilibrium, they can be represented in magnitude
and direction by the sides of a triangle taken in order. This is known as law of triangle of
forces.
The converse of this law is also true.
From the law of triangle of forces we get Lami's, theorem. If three forces acting at a
point, are in equilibrium, then each force is proportional to the sine of the angle between
the other two and conversely.
The magnitude of the resultant ?? of two like parallel forces ????
and ????
acting at ?? and ?? is
sum of their magnitudes, that is ?? i. ?? , and
the line of action of ????
(which is parallel to that of ?? and ?? ) divides ???? internally inversely
in the ratio of the forces ?? and ?? .
If two unlike parallel forces ????
and ????
act at ?? and ?? , the magnitude of their "resultant ????
is
?? Q and the tine of action of ????
divides ???? externally inversely in the ratio of the forces.
The moment about a point ?? of a force ???
whose line of action passes through a point ??
is ????
¯¯¯¯
×???
=???×???
If a number of forces act at a point, the algebraic sum of their (rectangular) components
along any direction is equal to the component of their resultant in the same direction; the
vector sum of their moments about any point is equal to the moment of their resultant
about the same point.
It follows that if a number of forces acting at a point are in equilibrium the algebraic sum
of their components along each of three mutually perpendicular directions must be zero
or the vector sum of their components about each of three non-collinear points must be
zero. Fic
If three forces are in equilibrium, then they must be co-planar and they must be either
concurrent or all of them must be parallel.
The resultant of two forces acting at a point O along ???? and ???? and represented in
magnitude by ?? : ?? A and ?? ,?? 3 is represented by (?? +?? )???? where ?? is a point on ????
such that ?? CA=?? ?? CB
A useful trigonometrical result
?? is a point on the side ?? B of a triangle ?????? such that ???? :???? =?? :??
Then
(?? +?? )cot?? =?? cot a-?? cot ??
(?? +?? )cot ?? =?? cos ?? -?? cot ??
Example 1
Forces acting in at a point I inside a quadrilateral ???????? represented by ???? ,???? ,???? and
?? D in magnitude and direction are in equilibrium. Find the position of ?? .
Let ?? and ?? be the midpoint of ???? and ???? respectively.
Then
PB
¯¯¯¯
·PC
¯¯¯¯
=2PN
¯¯¯¯
PD
¯¯¯¯
·PA
¯¯¯¯
=2PM
¯¯¯¯
We get
2(???
|???
+????
¯¯¯¯
)=????
¯¯¯¯
+???
+????
¯¯¯¯
+????
¯¯¯¯
=0 (given)
Therefore ?? is the midpoint of ?? ?? .
It is also the midpoint of the line joining the midpoints of ???? and ???? .
Example 2
A uniform circular plate of weight ?? is supported at three points in its edge whose
distances apart are a, b, c. Find the load carried by each support.
S (cicumcentre of, BC ) is the centre of the circuar plate. The weight ?? acts at ?? vertically
downward. Let ?? 1
,?? 2
,?? 3
be the forces acting vertically upward on the plate at ?? ,?? , C
respectively.
The forces acting on the plate are in equilibrium. Taking moments of the forces about the
line [???? , we get ?? 1
???? =?? ·?? ?? '
??? 1
:?? ·
?? ?? '
????
=?? ·
2cos ?? 2sin ?? .(?? is circum radius =SA )
(?
?? sin ?? =2?? and ?? 2
=?? 2
+?? 2
-2???? ·cos ?? )
=?? ?? 2
2cos ?? ????
=?? ?? 2
?? 2
?? 2
(?? 2
+?? 2
-?? 2
)
=
?? ?? 2
?? 2
?? 2
?? 2
[?? 2
(?? 2
+?? 2
-?? 2
)]
Therefore
?? 1
?? 2
(?? 2
+?? 2
-?? 2
)
=
?? 2
?? 2
(?? 2
+?? 2
-?? 2
)
=
?? 3
?? 2
(?? 2
+?? 2
-?? 2
)
=
?? 1
+?? 2
+?? 3
(?? +?? +?? )(?? +?? -?? ) (?? +?? -?? )(?? +?? -?? )
or ?? 1
=
?? ?? 2
(?? 2
+?? 2
-?? 2
)
(?? +?? +?? )(?? +?? -?? )(?? +?? -?? )(?? +?? -?? )
(??? 1
+?? 2
+?? 3
=?? )
Similar Expressions for ?? 2
and ?? 3
.
Example 3
A rod of length (?? +?? ) whose centre of gravity is at a distance a from one end remains
in (stable) - equilibrium in the vertical plane inside a fixed smooth isphere. Show that if ??
be the inclination of the rod to the horizontal then tan ?? =(
?? -?? ?? +?? )tan ?? , where 2?? is the
angle subtended by thie rod at the centre of the sphere:
The rod is acted by three forces, its weight W and the reactions ?? and ?? at its ends
and is in equilibrium.
Therefore
(?? +?? )cot (90-?? )=?? cot ?? -?? cot ??
or (?? +?? )tan ?? =?? cot (90-?? )-?? cot (90-?? )
=(?? -?? )tan ??
or tan ?? =(
?? -?? ?? +?? )tan ??
Example 4
Three forces ?? ,?? ,?? act along the sides BC, CA,AB of a triangle ABC . The resultant ties
along the line joining the centre of the circle inscribed in ?????? and the C.G. of the
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