3.4 Flashcards
Elasticity
Property of a body to resume its original shape or size once the deforming force or stress has been removed
Deformation
Change of shape or size of an object.
If the original shape returns the deformation is elastic;
If not it is plastic
Tensile vs compressive
Tensile forces are forces causing tension in the object
Compressive forces cause compression in the objects
Investigating stretching a wire PAG
Long thin wire firmly held by clamp at one end, other end supports a weight just heavy enough to keep the wire taut.
Marker placed on wire, distance from clamp to marker measured.
New weight added; new length to marker measured; process repeated.
Force plotted against extension
Limit of proportionality
The point at which the extension is no longer proportional to the force.
Elastic limit
Beyond this point the material will not show elastic behaviour: any more force added will lead to plastic deformation and the material will not return to its original shape
Fracture
The point at which the material will break due to excessive force
Hooke’s Law
The extension of an object is proportional to the force that causes it, provided the elastic limit is not exceeded.
Difference in loading and unloading energy.
When a force is applied to an object that causes deformation, the energy input in doing so is the loading energy.
If the deformation is plastic, when this force is removed, the object won’t return to its original shape and so the unloading energy will be less
Stress
The force per unit cross sectional area. σ
Strain
The extension per unit length
(Dimensionless) ε
Young modulus
Tensile stress/ tensile strain
E=σ/ε
Calculating Young modulus PAG
Searle’s apparatus:
Identical wires hung from support, one attached to a control weight, the other an extension weight.
Diameter of wire measured using micrometer, original length of wire measured.
New weight added and each time new wire extension measured. (Extension small so vernier calliper used)
Stress against strain plotted- gradient E
Ductile
Material can be drawn into wires.
Will show plastic deformation under tensile stress before breaking
Malleable
Can be hammered or beaten into flat sheets.
Will show extensive plastic deformation when compressed.
