Download What is force?

공학입문설계
서강대학교
전자공학과
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Chapter 10:
Force and Force-Related
Parameters
To surface, air is blown into the blast tank of a submarine to
push water out of the tank to make the submarine lighter – the
submarine floats to surface. To dive, the blast tank is opened
to let water in and to push out the air. The submarine sinks
below the surface. The diving and surfacing of the submarine
demonstrate the relationship between the weight of the
submarine and the buoyancy force acting on it.
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What We Mean by Force
What is force?
– The simplest form of a force that represents the interaction of
two objects is a push or a pull.
– The force is exerted by one body on another body by direct or
indirect contract
• Direct contract
• Indirect force: gravitation attraction
– Force is defined by
• Magnitude
• Direction
• Point of application
Units of force
– Newton’s law of motion
– 1 newton = (1 kg) (1 m/s2) or 1N = 1kg∙m/s2
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Spring Forces and Hooke｀s Law
Hooke’s law states that over the elastic range the
deformation of a spring is directly proportional to the
applied force, F = kx
where
F = applied force (N)
k = spring constant (N/mm or N/cm)
x = deformation of the spring (mm or cm)
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Example 10.1
What is the value of spring constant?
– k is the slope of a force-deflection line, k = 0.54 N/mm
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What We Mean by Force
Friction force – dry friction and viscous friction
– Dry friction exists because of irregularities between
surfaces in contact. F  N
max
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What We Mean by Force
Friction force – dry friction and viscous friction
– Fluid friction (viscous friction): a
measure of how easily the given fluid
can flow.
• The higher the viscosity value is, the
more resistance the fluid offers to flow.
– Honey vs. water
• The viscosity of fluids is a function of
temperature.
– In general, the viscosity of gases
increases with increasing temperature,
and the viscosity of liquids generally
decreases with increasing temperature.
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Newton’s Laws in Mechanics
First law (관성의 법칙)
– If a given object is at rest, and if there are no
unbalanced forces acting on it, the object will then
remain at rest.
– If the object is moving with a constant speed in a
certain direction, and if there are no unbalanced
forces acting on it, the object will continue to move
with its constant speed and in the same direction.
d
v0
dt
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Newton’s Laws in Mechanics
Second law of motion (가속도의 법칙)
– The net effect of unbalanced forces is equal to mass
times acceleration of the object, which is given by


the expression
F  ma

• A direct relationship between the force, the mass of
the object being pushed, and the acceleration of the
object.
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Newton’s Laws in Mechanics
Third law (작용 반작용의 법칙)
– For every action there exists a reaction, and the
forces of action and reaction have the same
magnitude and act along the same line, but they have
opposite directions.
Fab  Fba
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Newton’s Laws in Mechanics
Newton’s law of gravitation (만유인력의 법칙)
– Any two masses attract each other with a force that
is equal in magnitude and acts in the opposite
direction.
GM Earthm
GM Earth
W
, by letting g 
2
2
R
R
Gm1m2
Earth
Earth
F
W  mg
r2
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Pressure and Stress – Force Acting over
an Area
Pressure
– A measure of intensity of a force acting over an area.
force
pressure 
area
– Fluid pressure
• Hydrostatics, aerodynamics
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Pressure and Stress – Force Acting over
an Area
Common units of pressure
– Pascal
1N
1 Pa  2
m
– psi(pound per square inch)
2
 lb 
 lb  1ft 

P 2   P 2 
2 
 in 
 ft  144 in 
Fluid at rest
– Pascal’s law: the pressure of a fluid at a point is the
same in all directions, and
– The pressure of fluid increases with its depth
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Pressure and Stress – Force Acting over
an Area
Pascal’s law
– Pressure at a point is the same in all directions
– Pressure increases with the depth of fluid.
P  gh
– Buoyancy
FB  Vg
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Pressure and Stress – Force Acting over
an Area
Atmospheric pressure
– Due to the weight of the air
in the atmosphere above the
surface of the earth.
– At sea level: 101.325 kPa
– Commercial planes – 11,000m
• 1/5 of the sea-level
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Pressure and Stress – Force Acting over
an Area
Absolute pressure and gauge pressure
– Most pressure gauges show the magnitude of the
pressure of a gas or a liquid relative to the local
atmospheric pressure.
Pabsolute  Pgauge  Patmospheric
• Tire pressure gauge of 32 psi means that
the pressure of air inside the tire is 32 psi
above the local atmospheric pressure.
– Vacuum refers to pressures below atmospheric level.
• Absolute vacuum: no more air left in the container.
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Pressure and Stress – Force Acting over
an Area
Vapor pressure of a liquid
– Some fluids evaporate faster than others do.
• Alcohol has a higher vapor pressure than water.
– At a given temperature the fluids with low vapor
pressure require relatively smaller surrounding
pressure at their free surface to prevent them from
evaporating.
– Blood pressure
• Systolic pressure
• Diastolic pressure
• mmHg
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Pressure and Stress – Force Acting over
an Area
P1 
Hydraulic systems
F1
A1
F
– When F1 is applied, the piston 1 moves by
P2  2
A2
a distance L1, while piston 2 moves by
F F
a shorter distance, L2. A1 L1  A2 L2
P1  P2  1  2
– Relationship between
the speed of the pistons
V2 
A1
L2 
L1
A2
A1
F2 
A2
A2
F1
A1
A1
V1
A2
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Pressure and Stress – Force Acting over
an Area
Hydraulic systems (cont.)
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Pressure and Stress – Force Acting over
an Area
Stress
– A measure of the intensity of a force acting over an
area.
– Normal stress: the ratio of the normal(vertical)
component of the force to the area
– Shear stress: the ratio of the horizontal component of
the force to the area.
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Modulus of Elasticity, Modulus of Rigidity,
and Bulk Modulus of Compressibility
Modulus of elasticity (Young’s modulus)
– A measure of how easily a material will stretch when
pulled or how well the material will shorten when pushed.
– The larger the value,
the larger the force required.
where
E is the Young's modulus (modulus of elasticity)
F is the force applied to the object;
A0 is the original cross-sectional area through which the
force is applied;
ΔL is the amount by which the length of the object changes;
L0 is the original length of the object.
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Modulus of Elasticity, Modulus of Rigidity,
and Bulk Modulus of Compressibility
Modulus of rigidity (shear modulus)
– A measure of how easily a material can be twisted or
sheared.
– The measure show shows the resistance of a given
material to shear deformation.
• Steel is approximately three times more rigid in shear
when compared to aluminum.
where
= shear stress;
F is the force which acts
A is the area on which the force acts
= shear strain;
Δx is the transverse displacement
I is the initial length
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Modulus of Elasticity, Modulus of Rigidity,
and Bulk Modulus of Compressibility
Compressive strength
– The maximum compressive force per unit of the
cross-sectional area of the specimen.
Bulk modulus of compressibility
– It shows how easily the volume of the fluid can be
reduced when the pressure acting on it is increased.
– Pumping air into a bicycle tire
• Gases are more easily compressed than liquids
where
P is pressure, V is volume.
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Moment, Torque – Force Acting at a Distance
Two tendencies of an unbalanced force acting
on an object are to translate the object and to
rotate or bend or twist the object.
– Moment of a force
about an axis or a point.
M A  d1 F
M B  d2 F
MC  0
– Examples 10.14 and 10.15
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Work – Force Acting over a Distance
Mechanical work
– The component of the force that moves the object
times the distance the object moves.
W12  F cos d 
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Linear Impulse – Force Acting over Time
The concept is important in the design of air
bags and sport helmets.
– A stuntman jumping off a roof of a multistory building
onto an air mat on the ground.
• Increasing time of contact.
– Linear impulse
• The net effect of a force acting over a period of
time.



Faveraget  mV f  mVi
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