Lesson B3–3



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.)

Illinois Physical Science Applications in Agriculture Lesson B3–3 • Page 3

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 e. Shear

b. Stress f. Force

c. Strain g. Torsion

d. Tension

_______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?

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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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