Kinetics is the study of the relationship between forces acting on a system and its motion. It includes concepts like inertia, mass, force, weight, torque, and impulse. Forces can cause both acceleration and deformation of objects. Stress is the force distributed over an area, while pressure is the stress due to compression. Materials respond elastically to small loads but experience permanent plastic deformation above the yield point, with rupture occurring at ultimate failure. Repeated cyclic loading reduces the stress needed to cause material failure compared to a single acute load.

1. Subdivisions of Mechanics
Basic Concepts in Kinetics Kinematics
• the description of motion, including:
– considerations of space and time
– patterns and speeds of movement sequencing
Objectives: • the forces causing the motion are not considered
• Define basic concepts in kinetics, including:
inertia, mass, force, weight, torque, impulse, Kinetics
and stress • study of the relationship between the forces
• Define the different types of loading acting on a system and the motion of the system
• Learn how materials behave under loading
Inertia & Mass Center of Mass
(Center of Gravity)
Inertia
• Geometric point about which every particle of a
• concept relating to the difficulty with which an
body's mass is equally distributed
object’s motion is altered
• Position of the Center of Mass changes with
Mass changes in body configuration.
• the quantity of matter composing an object • Motion of the Center of Mass represents the
• the measure of inertia for linear motion “average” motion of the body as a whole
• the property giving rise to gravitational attraction
• Units:
– English: slug
– SI: kilogram (kg)
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2. Force Actions of Forces
• A mechanical interaction between an object and • Forces cause acceleration or deformation (a
its surroundings change in shape)
– We will assume that the forces acting on a body
• The “push” or “pull” of one object on another
cause minimal deformation
• Force is a vector. It has:
• Relationship between force (F), mass (m) and
– a magnitude
acceleration (a):
– a direction
– a point of application F F=ma
point of • Units:
application θ – English: pound (lb.) = (1 slug)(1 ft/s2)
– SI: Newton (N) = (1 kg)(1 m/s2)
– 1 lb. = 4.45 N
Net Force Concentrated vs. Distributed Force
• Resultant force derived from the composition of Concentrated Force
two or more forces • A force that is applied at a single point
• Reflects the net effect of all of the forces acting Distributed Force
together • A force that is applied over a distributed area
F4 • Can be approximated by a concentrated force
F1 that has the same net effect
Fnet F5
F5
F1 F4
F3 F2
F3 Fground
F2
Fground
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3. Weight Density & Specific Weight
• The force due to gravity (i.e. the pull of the Earth) • Volume : The amount of space occupied by a body.
• Weight has magnitude: Measured in (unit of length)3 (e.g. m3, ft3)
• Density (ρ): mass per unit volume:
W=mg
ρ = (mass)/(volume)
where:
m = mass SI Units: kg/m 3
g = acceleration due to
• Specific Weight : weight per unit volume
gravity (9.81 m/s2; 32.2 ft/s2)
(specific weight) = (weight)/(volume)
• Weight always acts at the W
center of mass and points English Units: lb./ft3
towards the center of the Earth SI Units: N/m3
Torque Impulse
• A measure of the extent to which a force will cause • The motion of a body depends not only on the force,
an object to rotate about a specific axis but also on the duration that the force is applied
• A net force applied through the center of mass
produces translation • Impulse : a measure related to the net effect of
• A net force applied away from the center of mass applying of force (F) for a time (t):
(i.e. an eccentric force) produces both translation
and rotation Impulse = F t
• Impulse increases with:
– Increased force magnitude
– Increased duration of application
F F • Equal impulses result in equal changes in velocity
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4. Compression, Tension, & Shear Stress & Pressure
• Compression : pressing or squeezing force • Stress: The force distributed over a given area:
directed normal (perpendicular) to a surface F
• Tension : pulling or stretching force directed σ= F
A
normal to a surface
• Shear : sliding or tearing force directed parallel where:
to a surface σ = stress
A
F = total force applied
Fn A = area force is applied over
Fn
Fs • Units:
– English: pounds per square inch (psi) = 1 lb./in 2
– SI: Pascal (Pa) = 1 N/m2
Compression Tension Shear • Pressure: stress due to a compressive force
Bending Torsion
• Asymmetric loading that produces tension on • Load producing a twisting of a body
one side of a body, compression on the other • Creates shear stresses
• Compressive and tensile stresses are greatest • Shear stresses are greatest at the surface
at the surface
F2
Compression
Tension
F3 Cross-section
F1 Cross-section
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5. Deformation Repetitive vs. Acute Loading
• Materials behave elastically at small loads • The size of the loading required to cause a material
• Loads above the yield point create permanent plastic to fail (i.e. fracture or rupture) decreases as the
deformation number of loading cycles increases
• Rupture or fracture occurs at the ultimate failure point
Stress Causing Failure
Yield Point
Ultimate
Elastic
Stress
Failure
Region Point Injury Likely To Occur
Plastic
Region
Deformation # of Loading Cycles
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