C6 - Springs and Hooke's Law Flashcards
What are compressive forces?
Forces that compress an object
This can cause compressive deformation
What is elastic deformation?
Deformation when the spring will return to its original length when the forces applied are removed
What is plastic deformation?
Deformation causing permanent structural changes to the spring so it does not return to its original length when the forces are removed
What is Hooke’s law?
The extension of the spring is directly proportional to the force applied provided the elastic limit of the spring is not exceed
How is Hooke’s law calculated?
F directly proportional to x
Or
F = kx
K is spring constant which measures the stiffness of a spring
X represents extension
How can Hooke’s law be investigated?
Using a spring and standard masses.
Attach the spring at one end using a clamp, boss and clamp stand secured to the bench using a G clamp.
Set up a metre ruler with the resolution of 1 mm close to the spring.
Suspend masses from the spring and record the total mass added a new length of spring and each mass.
A graph can help identify the law
How is work done by a spring calculated?
By the area under a force extension graph
E = 1/2 Fx
This leads to
E = 1/2 kx2
What is the force extension graph for a metal wire?
It follows hooks law until the elastic limit of the wire is reached.
The unloading graph will be identical for forces lesson on the elastic limit.
Beyond the elastic limit it is parallel to the loading graph but not identical as the wire is permanently extended after the force is removed
What is the first extension graph of rubber?
It does not obey Hooke’s law.
The band will return to its original length after the forces are removed however the loading and unloading graphs are both curved and different.
The loop formed is called a hysteresis loop which shows the amount of thermal energy released.
What is the force extension graph for polythene?
It does not obey Hooke’s law.
They are very easy to stretch, suffer plastic deformation and don’t return to their original size
What is tensile stress?
How is it calculated?
The force applied per unit of cross sectional area of the wire measured in Pa.
σ = F / A
What is tensile strain?
How is it calculated?
The fractional change in the original length of the wire. The extension per unit length.
ε = extension / original length ε = x / L
What is ultimate tensile strength?
The maximum stress a material can withstand when being stretched before it breaks.
Beyond this point the material may become longer and thinner at its weakest point, a process called necking.
Strong material is one with a high tensile strength.
The stress value at the point of fracture is known as the breaking strength.
What is the young modulus?
The ratio of stress to strain (when directly proportional to each other) for a particular material.
How is young modulus calculated?
Young modulus = stress / strain
E = σ / ε
