3.4.2.1 Bulk properties of solids Flashcards
What is density?
The compactness of a substance, mass it has per unit volume it takes up
What does density depend on?
The material an object is made from (not its shape or size)
How do you know whether an object will sink or float?
Depends on the average density of the substance - if the object is more dense than the fluid, it will sink, if it is less dense, it will float.
What is Hooke’s Law?
The extension of a stretched wire is proportional to the load (force applied). F=kdeltaL
What is k?
The stiffness of the object being stretched, also called the spring constant.
How can a force be applied to a spring?
Tensile force - stretch the spring
Compressive force - squash the spring
- Hooke’s law applies to both
What is the limit of proportionality/ Hooke’s law limit?
The point at which a material no longer obeys Hooke’s law when a force is applied
What is the elastic limit?
The point at which a material no longer returns to its original shape when the force is removed
What is elastic deformation?
If a material is elastically deformed, it returns back to its original shape when the force is removed
How does elastic deformation work?
When a material is put under tension, the atoms of the material are pulled apart from each other. They can move small distances relative to their equilibrium positions, without actually changing positions in the material. When the load is removed, atoms return to their equilibrium distance apart.
What is plastic deformation
The material no longer returns to its original shape when the force is removed - atoms don’t return to their original positions. A metal stretched past its elastic limit show plastic deformation
What is tensile stress?
The force applied per cross-sectional area (F/A)
What is tensile strain?
The change in length, i.e. deltaL/l (extension/original length)
What type of deformation forces cause strain/ stress?
Can be tensile or compressive
What is breaking stress?
When more and more stress is applied to a material it will cause atoms to move farther and farther apart. Eventually, the stress becomes so great that the atoms separate completely and the material breaks
What is UTS?
Ultimate Tensile Stress - the maximum stress that the material can withstand
What is elastic strain energy?
When a material is stretched, work has to be done in stretching the material. Before the elastic limit, all work done is stored as potential energy in the material, called elastic strain energy.
What is the formula for energy stored?
0.5FdeltaL
or 0.5k(deltaL)^2
What happens to energy when a spring is streched?
Energy is always conserved when stretching
Energy transfers in elastic deformation:
Work is done to stretch the material, if elastic deformation, all work done is stored as elastic strain energy in the material. When the stretching force is removed, the energy store is transferred to other forms
Energy transfers in plastic deformation:
Work is done to separate atoms, energy not stored as strain energy an mostly dissipated as heat
What are the energy changes in an oscillating spring?
Change in kinetic energy = change in potential energy
- potential energy includes both gravitational potential energy and elastic strain energy
How is energy conservation used in transport design?
Modern cars have crumple zones that deform plastically in a crash, meaning that some of the car’s kinetic energy goes into changing the chape of the vehicle’s body, so less transferred to the people inside. People more likely to survive but damage to the car is greater.
What is the stiffness of a material exactly?
A material’s ability to return back to its original shape after being subjected to a force
