Section 5: Materials Flashcards
What is density?
A measure of compactness of a substance. It relates the mass of a substance to how much space it takes up.
What is the equation for density?
P = m/v
Density (kgm^-3) = mass (kg) / volume (m^3)
What does the density of an object depend on?
What it’s made of. A materials density isn’t affected by shape or size.
What does the average density of an object determine?
Whether it floats or sinks
What is the density of water?
- 00 gcm^3
1. 00cm^3 of water has a mass of 1.00g
What is Hooke’s Law?
If a light metal wire with original length L is supported at the top and then had a weight attached to the bottom, it stretches. The weight pulls down with force F. Once the wire has stopped stretching, the forces will be equilibrium and there will be an equal and opposite reaction force at the support.
What did Robert Hooke discover in 1676?
That the extension of a stretched wire is proportional to the load force F. This relationship is called Hooke’s Law
What is the equation for Hooke’s Law?
F = k deltaL
Force (N) = stiffness constant (Nm^-1) x extension (m)
What does k depend on in Hooke’s Law?
The object being stretched
What does Hooke’s Law also apply to?
Springs
What is k usually described as?
The spring stiffness or the spring constant. It has the same value whether the forces are tensile or compressive
What does the force extension graph for Hooke’s Law show?
A straight line through the origin with the gradient representing k and the it curves off
Why does the graph curve off?
When the force becomes great enough, the graph starts to curve because it has reached the elastic limit
What is the elastic limit?
This is the point that if you carry on stretching the material it will be permanently stretched.
Where do metals generally obey Hooke’s Law to?
The limit of proportionality. It is very close to the elastic limit. It is the point at which the force is no longer proportional to the extension. It is also known as the Hooke’s Law limit
What experiment can you use to test how the extension of an object varies with the force used to extend it?
Attach a spring to a clamp stand with a rule from the bottom of the spring. Measure the length of the spring (original length) weights should then be added one at a time and the new length measured. After each weight is added the extension of the object can be calculated.
How can you calculate extension?
New length - original length
What does it mean if the object is elastic?
The material will return to its original length once the forces are removed. So it has no permanent extension
What does loading and unloading mean?
Loading means increasing the force on the material and unloading means reducing the force on the material
What happens when a material is put under tension?
The atoms of the material are pulled apart from one another
What happens if you stretch a plastic material?
The material will be permanently stretched after the force has been removed
How do plastic materials deform?
When the material is stretched, the atoms in the material move position relative to each other. When the load is removed, the atoms don’t return to their original positions. A Metal is stretched past its elastic limit shows plastic deformation
What’s the difference between tensile and compressive forces?
If the forces stretch the material they are tensile and if the forces squash the material they’re compressive forces