FORCE TYPES

Type of Forces

• when designing structures, there are two types of forces that must be considered

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External Force

• an external force is a force that �acts on a structure from the outside

GRAVITY

“NORMAL

FORCE”

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External Force

• an external force is a force that acts on a structure from the outside
• gravity is an external force that acts on all structures
• it is a non-contact force (acts on the structure without touching it)
• it always acts downward
• the force of gravity is determined by the mass of the object (mass and gravity together determine weight)

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External Force

• gravity is counteracted by the normal force
• the normal force is the force that surfaces exert on objects that are touching
• the normal force counteracts the force of gravity (it acts in the opposite direction and is the same magnitude)
• the normal force is a contact force (objects must be touching)

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Normal Force = Contact force

SURFACE 1

(BOX)

SURFACE 2

(TABLE)

REMEMBER LAST LESSON

“VECTORS”

SAME LENGTH = EQUAL MAGNITUDE

OPPOSITE DIRECTION

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STICKY SITUATION

PREDICT

OBSERVE

EXPLAIN

1) Interlace the pages of 2 sticky pads

2) Try to pull a part

PREDICT

OBSERVE

EXPLAIN

• When you pull, the individual sheets are put into TENSION
• Since the sheets are spread on an ANGLE
• this puts a HORIZONTAL and VERTICAL component of tension
• The harder you pull in the HORIZONTAL direction,
• the more pressure (tension) you are creating VERTICALLY to squish the pages together

PREDICT

OBSERVE

EXPLAIN

PREDICT

OBSERVE

EXPLAIN

Internal Force

• an internal force is a force that acts between two parts of a body
• four types of internal forces

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Tension (internal force)

• Tension pulls the particles of an object apart
• an object can only be stretched to it’s breaking point – after this point, the particles cannot stretch past each other and the object will snap
• an elastic band is put under tension when it is stretched but it will break if it is stretched too far

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Compression (internal force)

• Compression pushes the particles of an object together
• most compressed objects will return to their original shape when the compression force is removed
• e.g. a stress ball experiences compression when squeezed and returns to its original shape when released

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Torsion (internal force)

• Torsion is a twisting force in which one part of the object turns one way and the other part turns the other way or remains stationary
• e.g. when you wring out a wash cloth, you are creating torsion

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Shear (internal force)

• Shear forces occur when parts of the objects are pushed or pulled in opposite directions
• e.g. scissors use shear force to cut paper (one blade of the scissors pushes the paper one way and the other blade pushes the paper the other way)

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