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 BETWEENSTRESS 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?