Materials Flashcards
Define density
Density is the mass per unit volume
What is density a measure of?
Density is the measure of the compactness of a substance. It relates the mass of a substance to how much space it takes up
What is the formula for density?
Density = Mass/Volume
What Greek symbol is often used to represent density?
‘Rho’ (Looks like the letter p)
What are the units of density?
The units of density are grams per centimetre cubed or kilograms per metre cubed
What does the density of an object depend on?
The density of an object depends on what it is made of. Density does not vary with size or shape
What does the average density of an object determine?
The average density of an object determines whether it floats or sinks. A solid object will float on a fluid if it has a lower density than the fluid.
What is the density of water and what does this mean?
Water has a density of 1g per cm cubed. This means that 1cm cubed of water has a mass of 1g
What happens when a metal wire is supported at the top and then a weight is attached to the bottom?
If a metal wire is supported at the top and then a weight is attached to the bottom the wire stretches. The weight pulls down with a force of F producing an equal and opposite force at the support.
Define Hooke’s Law
Hooke’s law states that the extension of a stretched object is directly proportional to the load or force applied provided the limit of proportionality has not been exceeded.
How can Hooke’s Law be written as a formula?
Force = Spring Constant*Extension
What does ‘k’ mean in the formula for Hooke’s law?
K is a constant called the stiffness of an object that depends on the object being stretched
What is the relationship between springs and Hooke’s law?
Springs obey Hooke’s law, when you apply a pair of opposite forces the extension or compression of a spring is proportional to the force applied.
What is ‘k’ usually called for springs?
The spring constant
What is the relationship between Hooke’s law and compressive and tensile forces?
Hooke’s law works just as well for compressive forces as well as tensile forces. For a spring k has the same value whether the forces are tensile or compressive. This is not true for all materials.
What is the relationship between Hooke’s law and all other materials?
Hooke’s law doesn’t just apply to metal springs and wires. Most other materials obey it up to a point.
What happens to a spring when tensile forces are applied to it?
The spring is stretched
What happens to a spring when compressive forces are applied to it?
The spring is squashed
When does Hooke’s law stop working?
When force is great enough
What part of a graph shows that Hooke’s law is being obeyed?
When the graph is a straight line showing a straight line relationship between force and extension
Up to which point do metals generally obey Hooke’s law to?
The limit of proportionality
How can the limit of proportionality on a force extension graph be identified?
The limit of proportionality occurs when the graph starts to curve
What is the elastic limit of an material?
If you increase the load past the elastic limit the material will be permanently stretched. When all the force is removed the material will be longer than at the start.
What are the two different types of stretch?
Elastic and Plastic
What does it mean if a deformation is elastic?
If a deformation is elastic the material returns to its original shape and size once the forces are removed.
Explain the process of an elastic stretch
- When the 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 position in the material
- Once the load is removed the atoms return to their equilibrium distance apart
When does elastic deformation happen?
Elastic deformation happens as long as the elastic limit of an object is not reached
What does it mean if a deformation is plastic?
If a deformation is plastic the material is permanently stretched
Explain the process of a plastic stretch
- When the material is put under tension some atoms in the material move position relative to one another
- When the load is removed the atoms don’t return to their original positions
When does plastic deformation happen?
When an object has been stretched past its elastic limit