3.4 Materials Flashcards
what is the definition of Hooke’s law?
Hooke’s law states that the extension of an object is proportional to the force that causes it, provided that the elastic limit is not exceeded
what is the formula for Hooke’s law?
F = kx
where F is equal to the force
x is equal to the extension
k is equal to the force or spring constant
what does the force constant k dependent on?
the material being stretched
what can the force constant be used, or in other words when is F = kx valid?
when the material is undergoing ELASTIC deformation, as soon as the deformation becomes plastic, the force constant is no longer constant because the force per unit extension is not constant
what does the force constant k tell you?
the force per unit extension
what does Hooke’s law apply to?
wires being stretched and springs and most other materials
what is elasticity?
elasticity is the property of a body to resume to its original shape or size once the deforming force or stress has been removed
what are tensile forces?
stretching or pulling force on either end in opposite directions but equal size which increases the objects length (positive extension)
what are compressive forces?
compressive or squashing force
what is the gradient of a force-extension graph equal to?
k, the force or spring constant
what is the definition of deformation?
deformation is the change in shape or size of an object
what does elastic deformation mean?
the material returns back to its original shape when the deforming force is removed
what does plastic deformation mean?
the material does not return back to its original shape when the deforming force is removed, the deformation is plastic
outline an investigation to see force-extension characteristics for materials (not a wire)
- clamp the object being tested at the top and measure its original length using a ruler (could be a spring, rubber band, polythene strip etc.)
- ruler can either be held parallel or can be directly clamped
- add weights one at a time to the bottom of the object and record the new length
- calculate extension using = new length - original length
- plot a graph of force (weight) against extension, F = kx therefore gradient = k (before the object exceeds its limit of proportionality)
- make sure you carry it out safely, wear safety goggles and ensure there is a box underneath the weights to catch them
what does a line of constant gradient (linear relationship) on a force-extension graph tell you?
the material is obeying Hooke’s law
outline an investigation to see force-extension characteristics for a wire
- clamp a long, thin copper wire using a g clamp firmly at one end of the wire
- attach a pulley at the other with a weight hanger to keep the wire taught
- then attach a marker or mark on a position on the wire with a ruler in a fixed position lined up with the marker
- begin to add masses to the end of the wire at the pulley to stretch the wire
- record the mass of weights used to find the force
- record the extensions after each added mass
- plot a graph of force against extension
- make sure you carry it out safely, wear safety goggles and ensure there is a box underneath the weights to catch them
what is the area under a force-extension graph equal to?
the work done to extend that material by that amount (energy stored)