Transcript Force Tension Compression Shear and Torsion

Physical Science Applications
in Agriculture
Unit
Physical Science Systems
Problem Area
Agricultural Structural
Systems
Force:
Tension, Compression, Shear, Torsion
Lesson
What causes things to break?
• Have the students break a green branch
(approximately ½ to ¾ in diameter) over
their knee.
• Observe how the branch breaks.
• Why did the branch break in this way?
• What forces are at work?
Learning Objectives
1. Define forces and identify their effects.
2. Explain how tension, compression,
shear, and torsion differ.
3. Determine where tension, compression,
shear, and torsion are located in a
structural beam supported at both ends.
4. Recognize the relationship between
stress and strain.
Terms
Compression
Force
Shear
Strain
Stress
Tension
Torsion
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What is force and the effects of
forces?
• Force is the strength
or energy exerted
causing motion or
change of direction in
an object.
• If enough force is
applied on an object, it
will either be moved or
damaged.
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For example
• When an automobile crashes into a tree, the
force of the impact damages the automobile
and may break or damage the tree.
• These results will depend on the size and
force of the automobile.
• What are other examples of force?
• How can the understanding of force help in
the development of agriculture structures?
“Laws of Motions”
• Sir Isaac Newton
discovered three in
the 17th century.
• These laws are still
useful today.
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Newton’s first law
• States that if the forces of an object are in
balance, the object’s speed and direction of
motion will not change.
– If the forces on an object are in balance, the object’s
velocity is constant.
1. This simply means that if an object is not moving, the
object will stay still.
2. If the object is moving, it will continue in a straight line
at a constant speed.
3. What of the forces are not in balance? If the forces
are not in balance, then the object will accelerate,
which will increase or decrease speed and change
direction of the object.
NEWTON’S FIRST LAW
1. What forces are
acting?
2. What can you say
about the size of the
forces?
Newton’s second law
•
States how much an object accelerates if
the forces are unbalanced.
A. If an unbalanced force is applied to an
object, it will accelerate.
B. If twice the force is applied, there will be
twice the acceleration.
C. If the same force is applied to an object
with twice the mass, there will be only half
the acceleration
Newton’s third law
• Explains that every action has an equal
or opposite reaction.
– For example, when one pushes or pulls
something, it pushes or pulls back.
– Forces are “two-way” associations.
– Give an example of how this law works.
FORCE AND ITS EFFECTS
•
Forces can change:
–
–
–
The speed of an object
The direction that an object is moving in
The shape of an object
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How do tension, compression,
shear, and torsion differ?
• Stress is a force acting on a material
causing it to change shape.
• The stresses of structural design are
tension, compression, shear, and
torsion.
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How do tension, compression,
shear, and torsion differ?
A. Tension occurs when opposing forces are
pulling to stretch or elongate an object.
– Tensile forces produce internally in structural
materials.
B. Compression forces occur when pushing
on an object to shorten or compress it.
– Compression and tension are normal stresses.
– The stressed surface is perpendicular to the
applied force.
How do tension, compression,
shear, and torsion differ?
C. Shearing forces act opposite to one
another.
– Think of how a pair of scissors works.
– The direction of shearing stress is
different than compression and tension
because the surface that is being applied
stress is parallel to the direction of force.
How do tension, compression,
shear, and torsion differ?
D. Torsion is a rotating force.
E. All of these stresses, tension,
compression, shear, and torsion are
very common in building structures.
– Each one of these stresses must be fully
understood before a structure can be
designed and constructed.
– Materials and safety measures must also
be considered in terms of these stresses
NORMAL AND SHEARING
STRESSES
Where do tension, compression,
shear, and torsion occur in a
beam supported on both ends?
• When one can understand the location
of the stress, then knowledge can be
applied to the design and construction
of building materials.
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TENSION, COMPRESSION,
SHEAR, AND TORSION IN A
BEAM LOADED AT ITS CENTER
Think of the green branch that
was broken earlier.
• When the branch was bent over the knee,
what force is applied to the surface opposite
of the knee?
• What happens to this surface?
• What force is applied to the surface in contact
with the knee? (surface that wrinkles)
• What happens between the two surfaces?
Why does the branch split?
A. Tension occurs on the bottom of the beam.
B. Compression occurs on the top side of the
beam.
C. Shear occurs at the points over the supports
and horizontally through the beam.
D. Torsion occurs at the points where the beam
rotates downward around the supports.
What is the relationship between
stress and strain?
• As loads are added
to a beam, the beam
will develop internal
stresses and the
beam will be
deflected.
• Strain is the
deformation of a
material under the
action of applied
forces.
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What is the relationship between
stress and strain?
• A strain is a result of a stress.
– Stresses will produce strains
proportionately until the proportional limit is
reached.
– Ultimately, the strain will become so great,
the material will fail.
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What is the relationship between
stress and strain?
• Structures are designed to exceed
known and assumed design loads.
– For safety purposes, materials are not
used to their full strength when designing
machines or structures.
– How safe does the structure need to be?
RELATIONSHIP BETWEEN
STRESS AND STRAIN
Review/Summary
• What is force and the effects of forces?
• How do tension, compression, shear, and
torsion differ?
• Where do tension, compression, shear, and
torsion occur in a beam supported on both
ends?
• What is the relationship between stress and
strain?