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STATICS
Chapter 2
CONCEPT OF FORCE AND
EQUILIBRIUM
• Statics – study on body that is at rest or in
equilibrium
• Force – a push or a pull exerted by our
muscles (primitive concept)
- an interaction that causes changes in the
motion of bodies
- action of one body on another
CONCEPT OF FORCE AND
EQUILIBRIUM
• If the interaction between bodies
does not produce motion, it means
that the forces neutralize each other.
• If forces are not neutralized, a
change in motion of the body or
system will result.
CLASSIFICATIONS OF FORCES
• Concurrent – forces that act at a
point or whose line of action
converges or intersects at a common
point
• Nonconcurrent – forces whose line
of action does not converge at a
common point
CLASSIFICATIONS OF FORCES
• External force – force that a body
exerts on another body
• Internal force – forces exerted by one
part of a body on other parts of
same body
CLASSIFICATIONS OF FORCES
• Co-planar – forces acting on one
plane
• Non-coplanar – forces acting in more
than one plane
UNITS OF FORCE
• (MKS) Newton = force that will give a
mass of 1 kg an acceleration of 1 m/s2
• (CGS) dyne = force that will give a mass
of 1 g an acceleration of 1 cm/s2
• (FPS) poundal = force that will give a
mass of 1 slug an acceleration of 1 ft/s2
UNITS OF FORCE
•
•
•
•
1 kg force = 9.8 N
1 g force = 980 dynes
1 lb force = 32 poundal
1 slug mass = 32 lb
RESULTANT AND COMPONENT OF
FORCES
• Forces are vector quantities
• Composition of forces = process of finding a
single force which will produce an effect the
same as the effect produced by the given
forces
• Resolution of forces = a single force is broken
into separate forces called the component of
the force.
EQUILIBRIUM OF PARTICLES
• A particle which has no net force acting
to it is said to be in equilibrium.
• Equilibrant – single force that holds two
or more forces in equilibrium. It prevents
the motion of the body which is equal to
the resultant but in opposite direction.
EQUILIBRIUM OF PARTICLES
• Equilibrium – state in which there is no
change in the motion of the body
– Static equilibrium – resultant of all the
forces is zero and the body is at rest
– Dynamic equilibrium – resultant of all forces
is zero, velocity of body is constant thus
body moves in uniform motion
FIRST CONDITION FOR EQUILIBRIUM
𝐹=0
• If the resultant of all the forces acting
on a body is zero, the sum of the
rectangular components of these
forces along any axis must be zero.
FIRST CONDITION FOR EQUILIBRIUM
• The sum of all upward force
components is equal to the sum of
all downward force components.
• The sum of all force components to
the right is equal to the sum of all
force components to the left.
EXAMPLE
A pendulum bob with a weight of 20 N
hangs from a cord. A horizontal force
sufficient to bring the cord to an angle
of 25° with the vertical is applied to
the bob. Find the horizontal force and
the tension in the cord.
SECOND CONDITION FOR
EQUILIBRIUM
• When vector sum of concurrent
forces acting on a body is zero, then
there is no change in its translational
motion but when there are several
non-concurrent forces acting on a
body, there is in general a change in
the state of rotational motion.
SECOND CONDITION FOR
EQUILIBRIUM
• In order that the rotational motion of
a body will not change, the sum of
the torques about any axis acting on
the body must be equal to zero.
SECOND CONDITION FOR
EQUILIBRIUM
𝜏=0
Clockwise torque = Counterclockwise
torque
TORQUE OR MOMENT OF FORCE
• Measures the effectiveness of the
force in changing rotation about the
chosen axis.
• Turning effect of the force.
TORQUE OR MOMENT OF FORCE
• The effect of a force on a rotational
motion of a body depends on:
–Magnitude of the force
–Torque arm or moment arm
• Perpendicular distance from the axis of
rotation (fulcrum) to the line of action of
the force
TORQUE OR MOMENT OF FORCE
• To change the state of rotational motion
of a body, the point of application of the
force is taken into consideration.
• The effect of a given force upon the
rotational motion of a body is greater,
the further the line of action is from the
axis of rotation.
TORQUE OR MOMENT OF FORCE
• If the line of action of the applied
force passes through the axis of
rotation, then this force will not
produce any change in the rotational
motion of the body.
CENTER OF GRAVITY
• Point about which the sum of gravitational
torques is equal to zero
• Point of application of the resultant of the
attraction that the earth exerts upon all the
particles of a body
• A point in which the total weight of the body
is concentrated
• A point where object balances
CENTER OF GRAVITY
• The center of gravity of a body may
be situated outside of the body.
• Center of gravity determines the
stability of the body.
CENTER OF GRAVITY
• When a force whose line of action passes
through the center of gravity, this force
will affect only the translational motion
of the body but when line of action does
not pass through the center of gravity
both translational and rotational motion
of the body is affected.
TYPES OF EQUILIBRIUM
• Stable – center of gravity at its lowest
position, object displaced slightly will
return to original position
• Unstable – object when displaced slightly
will change position completely
• Neutral – object when displaced, location
of center of gravity remains the same