Flashcards in Materials Deck (87)
What is the density of a material?
Its mass per unit volume
What is the equation for density?
Density = mass / volume
What 1 g cm^-3 equal to?
1000 kg m^-3
Why does oil float on water?
It has a lower density than water
Why will a solid object float on a fluid?
If its density is less than the density of the fluid
What is Hooke's Law?
The extension of a stretched wire is directly proportional to the force applied
What happens, in terms of forces, when a light metal wire is supported at the top and then has a weight attached at the bottom?
The weight pulls down with a force, stretching the wire. Once the wire stops stretching, the forces will be in equilibrium and there will be an equal and opposite reaction force at the support
What is an objects stiffness constant?
The force needed to extend it by 1m
What is the equation for Hooke's Law?
Force = stiffness constant x extension
What does a tensile force do?
What does a compressive force do?
What are 2 features of a force-extension graph that shows the material obeys Hooke's Law?
It's a straight line through the origin
What is the gradient of a force-extension graph equal to?
At what point on a force-extension graph does an object reach its elastic limit?
When the graph starts to curve
What happens if you increase the force past the elastic limit?
The material will be permanently stretched (and won't go back to original shape when force is removed)
What is the elastic limit of an object?
The force beyond which a material will be permanently stretched
What is the limit of proportionality?
The point beyond which the force is no longer proportional to the extension (so the gradient of a force-extension graph is no longer linear)
On a force-extension graph, where do you mark the limit of proportionality?
When the line of the graph is no longer straight
What are 5 the steps for the experiment to investigate the extension?
-Set up a clamp stand and clamp with a spring attached
-Take a measurement of the original length of the spring
-One at a time, add weights to the bottom of the spring, and measure the new length
-Calculate extension by doing extension = new length - original length
-Plot a graph of force against extension
When investigating the extension of a spring in a lab, why do you trial investigation first?
So you can work out the size and range of weights to get lots of measurements before the objects break
What does loading mean?
Increasing the force on the material
What does unloading mean?
Reducing the force on the material
What is elastic deformation?
Where the material returns to it's original shape once the forces are removed - there's no permanent extension
How can you tell a material is elastic from a force-extension curve showing loading and unloading?
If both lines start and end in the same position
What happens when a material is put under tension?
The atoms of the material are pulled apart from one another, as atoms can move small distances from their equilibrium position without changing position in the material
What happens to the atoms in a material once a force is removed?
The atoms return to their equilibrium distance apart
For a metal, elastic deformation happens as long as...?
Hooke's Law is obeyed
What is plastic deformation?
Where the material is permanently stretched after the force is removed
What happens to the atoms in a material when plastic deformation occurs?
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.