# 12.2 Examples of static equilibrium  (Page 8/9)

 Page 8 / 9

We select the pivot at point P (upper hinge, per the free-body diagram) and write the second equilibrium condition for torques in rotation about point P :

$\text{pivot at}\phantom{\rule{0.2em}{0ex}}P\text{:}\phantom{\rule{0.2em}{0ex}}{\tau }_{w}+{\tau }_{Bx}+{\tau }_{By}=0.$

We use the free-body diagram to find all the terms in this equation:

$\begin{array}{ccc}\hfill {\tau }_{w}& =\hfill & dw\phantom{\rule{0.2em}{0ex}}\text{sin}\left(\text{−}\beta \right)=\text{−}dw\phantom{\rule{0.2em}{0ex}}\text{sin}\phantom{\rule{0.2em}{0ex}}\beta =\text{−}dw\frac{b\phantom{\rule{0.1em}{0ex}}\text{/}\phantom{\rule{0.1em}{0ex}}2}{d}=\text{−}w\frac{b}{2}\hfill \\ \hfill {\tau }_{Bx}& =\hfill & a{B}_{x}\text{sin}\phantom{\rule{0.2em}{0ex}}90\text{°}=+a{B}_{x}\hfill \\ \hfill {\tau }_{By}& =\hfill & a{B}_{y}\text{sin}\phantom{\rule{0.2em}{0ex}}180\text{°}=0.\hfill \end{array}$

In evaluating $\text{sin}\phantom{\rule{0.2em}{0ex}}\beta ,$ we use the geometry of the triangle shown in part (a) of the figure. Now we substitute these torques into [link] and compute ${B}_{x}:$

$\text{pivot at}\phantom{\rule{0.2em}{0ex}}P\text{:}\phantom{\rule{0.2em}{0ex}}\text{−}w\phantom{\rule{0.1em}{0ex}}\frac{b}{2}+a{B}_{x}=0\phantom{\rule{0.5em}{0ex}}⇒\phantom{\rule{0.5em}{0ex}}{B}_{x}=w\phantom{\rule{0.1em}{0ex}}\frac{b}{2a}=\left(400.0\phantom{\rule{0.2em}{0ex}}\text{N}\right)\phantom{\rule{0.1em}{0ex}}\frac{1}{2·2}=100.0\phantom{\rule{0.2em}{0ex}}\text{N.}$

Therefore the magnitudes of the horizontal component forces are ${A}_{x}={B}_{x}=100.0\phantom{\rule{0.2em}{0ex}}\text{N}.$ The forces on the door are

$\begin{array}{}\\ \text{at the upper hinge:}\phantom{\rule{0.2em}{0ex}}{\stackrel{\to }{F}}_{A\phantom{\rule{0.2em}{0ex}}\text{on door}}=-100.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{i}+200.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{j}\hfill \\ \text{at the lower hinge:}{\stackrel{\to }{F}}_{B\phantom{\rule{0.2em}{0ex}}\text{on door}}=\text{+}100.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{i}+200.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{j}.\hfill \end{array}$

The forces on the hinges are found from Newton’s third law as

$\begin{array}{}\\ \\ \text{on the upper hinge:}\phantom{\rule{0.2em}{0ex}}{\stackrel{\to }{F}}_{\text{door on}\phantom{\rule{0.2em}{0ex}}A}=100.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{i}-200.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{j}\hfill \\ \text{on the lower hinge:}\phantom{\rule{0.2em}{0ex}}{\stackrel{\to }{F}}_{\text{door on}\phantom{\rule{0.2em}{0ex}}B}=-100.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{i}-200.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{j}.\hfill \end{array}$

## Significance

Note that if the problem were formulated without the assumption of the weight being equally distributed between the two hinges, we wouldn’t be able to solve it because the number of the unknowns would be greater than the number of equations expressing equilibrium conditions.

Check Your Understanding Solve the problem in [link] by taking the pivot position at the center of mass.

${\stackrel{\to }{F}}_{\text{door on}\phantom{\rule{0.2em}{0ex}}A}=100.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{i}-200.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{j}\phantom{\rule{0.2em}{0ex}}\text{;}\phantom{\rule{0.2em}{0ex}}{\stackrel{\to }{F}}_{\text{door on}\phantom{\rule{0.2em}{0ex}}B}=-100.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{i}-200.0\phantom{\rule{0.2em}{0ex}}\text{N}\stackrel{^}{j}$

Check Your Understanding A 50-kg person stands 1.5 m away from one end of a uniform 6.0-m-long scaffold of mass 70.0 kg. Find the tensions in the two vertical ropes supporting the scaffold.

711.0 N; 466.0 N

Check Your Understanding A 400.0-N sign hangs from the end of a uniform strut. The strut is 4.0 m long and weighs 600.0 N. The strut is supported by a hinge at the wall and by a cable whose other end is tied to the wall at a point 3.0 m above the left end of the strut. Find the tension in the supporting cable and the force of the hinge on the strut.

1167 N; 980 N directed upward at $18\text{°}$ above the horizontal

## Summary

• A variety of engineering problems can be solved by applying equilibrium conditions for rigid bodies.
• In applications, identify all forces that act on a rigid body and note their lever arms in rotation about a chosen rotation axis. Construct a free-body diagram for the body. Net external forces and torques can be clearly identified from a correctly constructed free-body diagram. In this way, you can set up the first equilibrium condition for forces and the second equilibrium condition for torques.
• In setting up equilibrium conditions, we are free to adopt any inertial frame of reference and any position of the pivot point. All choices lead to one answer. However, some choices can make the process of finding the solution unduly complicated. We reach the same answer no matter what choices we make. The only way to master this skill is to practice.

## Conceptual questions

Is it possible to rest a ladder against a rough wall when the floor is frictionless?

Show how a spring scale and a simple fulcrum can be used to weigh an object whose weight is larger than the maximum reading on the scale.

(Proof)

show whether or not the expression v^2= u^2 sin^2 d- 2gs is dimensionally constant
the period T of a pendulum depends on its mass m, length l and acceleration due to gravity g. using dimensional analysis, derive for T.
Oyetayo
what is physics
Physics is the tool humans use to understand the properties characteristics and interactions of where they live - the universe. Thus making laws and theories about the universe in a mathematical way derived from empirical results yielded in tons of experiments.
Jomari
This tool, the physics, also enhances their way of thinking. Evolving integrating and enhancing their critical logical rational and philosophical thinking since the greeks fired the first neurons of physics.
Jomari
nice
Satyabrata
Physics is also under the category of Physical Science which deals with the behavior and properties of physical quantities around us.
Angelo
Physical Science is under the category of Physics*... I prefer the most is Theoretical Physics where it deals with the philosophical view of our world.
Jomari
what is unit
Metric unit
Arzoodan
A unit is what comes after a number that gives a precise detail on what the number means. For example, 10 kilograms, 10 is the number while "kilogram" is the unit.
Angelo
there are also different types of units, but metric is the most widely used. It is called the SI system. Please research this on google.
Angelo
Unit? Bahay yon
Jomari
How did you get the value as Dcd=0.2Dab
Why as Dcd=0.2Dab? where are you got this formula?...
Arzoodan
since the distance Dcd=1.2 and the distance Dab=6.0 the ratio 1.2/6.0 gives the equation Dcd=0.2Dab
sunday
Well done.
Arzoodan
how do we add or deduct zero errors from result gotten using vernier calliper?
how can i understand if the function are odd or even or neither odd or even
hamzaani
I don't get... do you mean positive or negative@hamzaani
Aina
Verner calliper is an old calculator
Antonio
Function is even if f(-x) =f(x)
Antonio
Function is odd if f(-x) = - f(x)
Antonio
what physical phenomena is resonance?
is there any resonance in weight?
amrit
Resonance is due to vibrations and waves
Antonio
wait there is a chat here
dare
what is the difference between average velocity and magnitude of displacement
ibrahim
how velocity change with time
ibrahim
average velocity can be zero positive negative but magnitude of displacement is positive
amrit
if there is different displacement in same interval of time
amrit
Displacement can be zero, if you came back
Antonio
Displacement its a [L]
Antonio
Velocity its a vector
Antonio
Speed its the magnitude of velocity
Antonio
[Vt2-Vt1]/[t2-t1] = average velocity,another vector
Antonio
Distance, that and only that can't be negative, and is not a vector
Antonio
Distance its a metrical characteristic of the euclidean space
Antonio
Velocity change in time due a force acting (an acceleration)
Antonio
the change in velocity can be found using conservation of energy if the displacement is known
Jose
BEFORE = AFTER
Jose
kinetic energy + potential energy is equal to the kinetic energy after
Jose
the potential energy can be described as made times displacement times acceleration. I.e the work done on the object
Jose
mass*
Jose
from there make the final velocity the subject and solve
Jose
If its a conservative field
Antonio
So, no frictions in this case
Antonio
right
Jose
and if still conservative but force is in play then simply include work done by friction
Jose
Is not simple, is a very unknown force
Antonio
the vibration of a particle due to vibration of a similar particle close to it.
Aina
No, not so simple
Antonio
Frequency is involved
Antonio
mechanical wave?
Aina
All kind of waves, even in the sea
Antonio
will the LCR circut pure inductive if applied frequency becomes more than the natural frequency of AC circut? if yes , why?
LCR pure inductive? Is an nonsense
Antonio
what is photon
Photon is the effect of the Maxwell equations, it's the graviton of the electromagnetic field
Antonio
a particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass.
Areej
Quantum it's not exact, its the elementary particle of electromagnetic field. Its not well clear if quantum theory its so, or if it's classical mechanics improved
Antonio
A photon is first and foremost a particle. And hence obeys Newtonian Mechanics. It is what visible light and other electromagnetic waves is made up of.
eli
No a photon has speed of light, and no mass, so is not Newtonian Mechanics
Antonio
photon is both a particle and a wave (It is the property called particle-wave duality). It is nearly massless, and travels at speed c. It interacts with and carries electromagnetic force.
Angelo
what are free vectors
a vector hows point of action doesn't static . then vector can move bodily from one point to another point located on its original tragectory.
Anuj
A free vector its an element of an Affine Space
Antonio
Clay Matthews, a linebacker for the Green Bay Packers, can reach a speed of 10.0 m/s. At the start of a play, Matthews runs downfield at 45° with respect to the 50-yard line and covers 8.0 m in 1 s. He then runs straight down the field at 90° with respect to the 50-yard line for 12 m, with an elapsed time of 1.2 s. (a) What is Matthews’ final displacement from the start of the play? (b) What is his average velocity?
Clay Matthews, a linebacker for the Green Bay Packers, can reach a speed of 10.0 m/s. At the start of a play, Matthews runs downfield at 45Â° with respect to the 50-yard line and covers 8.0 m in 1 s. He then runs straight down the field at 90Â° with respect to the 50-yard line for 12 m, with an elap
ibrahim
Very easy man
Antonio
how to find time moved by a mass on a spring
Maybe you mean frequency
Antonio
why hot soup is more tastier than cold soup?
energy is involved
michael
hot soup is more energetic and thus enhances the flavor than a cold one.
Angelo
Its not Physics... Firstly, It falls under Anatomy. Your taste buds are the one to be blame not its coldness or hotness. Secondly, it depends on how the soup is done. Different soups possess different flavors and savors. If its on Physics, coldness of the soup will just bore you and if its hot...
Jomari
what is the importance of banking road in the circular path
the coefficient of static friction of the tires and the pavement becomes less important because the angle of the banked curve helps friction to prevent slipping
Jose
an insect is at the end of the ring and the ring is rotating at an angular speed 'w' and it reaches to centre find its angular speed.
Angular speed is the rate at which an object changes its angle (measured) in radians, in a given time period. Angular speed has a magnitude (a value) only.  v represents the linear speed of a rotating object, r its radius, and ω its angular velocity in units of radians per unit of time, then v = rω
Angular speed = (final angle) - (initial angle) / time = change in position/time. ω = θ /t. ω = angular speed in radians/sec.
a boy through a ball with minimum velocity of 60 m/s and the ball reach ground 300 metre from him calculate angle of inclination