Download psaa forces worksheet

FORCE: TENSION,
COMPRESSION, SHEAR, AND
TORSION
Student Learning Objectives. Instruction in this lesson should result in
students achieving the following 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.
Anticipated Problem: What is force and the effects of forces?
I. 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. 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?
A. Sir Isaac Newton discovered three “Laws of Motions” in the 17th century. These laws
are still useful today.
1. 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.
a. This simply means that if an object is not moving, the object will stay still.
b. If the object is moving, it will continue in a straight line at a constant speed.
c. 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.
2. Newton’s second law stated 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.
3. 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. (Think of sitting
down in a chair.)
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Anticipated Problem: How do tension, compression, shear, and torsion differ?
II. Stress is a force acting on a material causing it to change shape. The stresses of
structural design are tension, compression, shear, and torsion.
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.
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.
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.
Anticipated Problem: Where do tension, compression, shear, and torsion occur in a
beam supported on both ends?
III. When one can understand the location of the stress, then knowledge can be applied to
the design and construction of building materials. Think of the green branch that was
broken earlier within the lesson. When we 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.
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Anticipated Problem: What is the relationship between stress and strain?
IV. 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.
A. 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.
B. 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?
FORCE: TENSION, COMPRESSION,
SHEAR, AND TORSION
Part One: Matching
Instructions: Match the word with the correct definition.
a. Compression
b. Stress
c. Strain
d. Tension
e. Shear
f. Force
g. Torsion
_______1. Force acting on a material causing it to change shape
_______2. Rotating force
_______3. Opposing forces pulling on a object to stretch or elongate
_______4. A load applied to an object tending to change shape, position, and speed
_______5. Forces pushing on an object to shorten or compress it
_______6. Amount of bending of a material under a load
_______7. Forces acting opposite one another
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements.
1. _________ occurs at the bottom of a beam
2. A __________ is a result of stress.
3. If you have an unbalance force to an object, it will _______________.
4. These stresses are applied to an object perpendicular ________________ and
__________________.
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Part Three: Multiple Choice
Instructions: Write the letter of the correct answer.
_______1. If the forces of an object are in balance, the object’s speed and direction of the motion
will not change.
a. Newton’s first law
b. Newton’s second law
c. Newton’s third law
d. Law of acceleration
_______2. Stress being applied to an object is parallel to the direction of force
a. Compression
b. Tension
c. Torsion
d. Shear
_______3. Newton’s second law stated
a. If an object is not moving, the object will stay still.
b. How much an object accelerates if the forces are unbalanced.
c. Every action has an equal or opposite reaction.
d. If the forces of an object are in balance, the object’s speed and direction of the motion will not
change.
_______4. Occurs horizontally through the beam.
a. Tension
b. Compression
c. Shear
d. Torsion
Part Four: Short Answer
Instructions: Answer the following questions.
1. What is a force?
2. How do tension, compression, shear, and torsion differ?
3. What is the relationship between stress and strain?
Illinois Physical Science Applications in Agriculture Lesson B3–3 • Page 8
Assessment
TS–A
Technical Supplement
FORCE: TENSION,
COMPRESSION, SHEAR, AND
TORSION
1. What is a force?
Force can be mathematically described as mass times acceleration. Often in the form
of a push or a pull, force is any influence that can cause a massive body to be accelerated
(deformed). Sources of forces include gravity, electricity, magnets, or simple muscular
effort.
2. What are the effects of force?
Force causes acceleration. Acceleration is defined as the rate at which a body’s velocity
changes with time, in magnitude or direction.
3. How do tension, compression, shear, and torsion differ?
Tension is the presence of opposing forces causing a body to stretch or elongate.
Compression is the result of opposing forces pushing on a body to shorten it. Shear
is a deformation along a plane or set of planes tangential to the applied force, the
result of forces acting on a body like scissors. Torsion is the result of a rotating force.
4. Where do tension, compression, shear, and torsion occur in a beam supported
on both ends?
Tension occurs on the bottom of the beam. Compression occurs on the top side of
the beam. Shear occurs at the points over the supports and horizontally through the
beam. Torsion occurs at the points where the beam rotates downward around the
supports.
5. How can we modify design to balance and/or control these forces?
Modifying factors have been used for thousands of years. For instance, in the
construction of the Acropolis, the Grecians used extra support columns along the
length of the horizontal beams for the roof. An extra column of the point of tension
would then counteract the effects of compression. An advance on this design uses arches
which deflect the compressive force, thereby allowing the builder to use less supporting
columns. Stone and brick bridges and buildings use this concept. In buildings, this is a
good way to add windows, because the compressive forces are deflected away from the
weak point, that is, the hole in the wall. Ironically, placing a load on a properly arched
structure increases its strength. In an arch, compressive forces compact the materials and
hold them together, even with the absence of cement, as in the case of old stone bridges.
Modern day arenas, such as the Astrodome, are designed with dome roofs to eliminate
vertical columns that become visual impairments. These domes are a series of arches
circling the base, compressed together by their weight. The compression causes the
weight to be deflected and keeps the underside of the roof from being torn apart from
tensional forces.
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