Section 5 Materials Flashcards
What is density?
It is a measure of the ‘compactness’ of a substance.
The material’s mass per unit volume
What is the equation of density?
p = m/V
What does the density of an object depend on?
The material it is made of
What is Hooke’s Law?
The applied force on the spring is directly proportional to the extension of the spring, provided the spring has not been extended beyond its limit of proportionality.
What is the equation for the pressure on solids?
P = F/A
What is the equation for pressure in fluids?
P = pgh
What is the equation for Hooke’s Law?
F = kx
What is the spring constant?
The constant of proportionality for the extension of a spring under a force. The higher the spring constant, the greater the force needed to achieve a given extension.
The extension or compression of a spring is proportional to what?
the force applied
How do you know if a spring is obeying Hooke’s Law from a force-extension graph?
There should be a straight line through the origin
On a force-extension graph, what is the gradient?
the spring constant , k
What is the elastic limit of a spring?
When the force becomes great enough, the force-extension graph starts to curve. This point on the graph is the elastic limit.
What happens if you keep applying force past the elastic limit?
The material will be permanently deformed. When all the force is removed, the material will no longer than at the start.
What is another name for the limit of proportionality?
Hooke’s law limit
Where is the limit of proportionality on a force-extension graph?
It is the point beyond which the force is no longer proportional to extension.
It is just before the elastic limit.
It is where the straight line ends and starts to curve.
What is the equation for extension?
new length - original length
How can you investigate the extension of springs?
The object under test should be supported at the top, using a clamp, and a measurement of its original length taken using a ruler. Weights should then be added one at a time to the other end of the object.
The weights used will depend on the object being tested - you should do a trial investigation if you can to work out the range and size of weights needed. You want to be able to add the same size weights each time and add a large number of weights before the object breaks, to get a good picture of how the extension of the object varies with the force applied to it.
After each weight is added, the extension of the object can be calculated.
Finally, a graph of load against extension should be plotted to show the results.
What does it mean if a deformation is elastic?
The material returns to its original shape once the forces are removed - so it has no permanent extension.
How can you tell if a material is elastic from a force-extension graph?
It has 2 different curves. The top one being loading and the bottom one being unloading. It also returns to its original length after the extension.
What happens to the atoms in a material when this material is put under tension and when this load is then removed?
When a material is put under tension, the atoms of the material are pulled apart from one another. Atoms can move small distances relative to their equilibrium positions without actually changing positions in the material. Once the load is removed, the atoms return to their equilibrium distances apart.
What are tensile forces?
When the forces stretch the material
What are compressive forces?
When the forces squash the material