6. Materials Flashcards
(26 cards)
What are tensile forces?
Forces that produce extension
What are compressive forces?
Forces that shorten an object
What is Hooke’s law?
The extension of the spring is directly proportional to the force applied.
This is true as long as the elastic limit of the spring is not exceeded
Describe the characteristics of a force-extension graph for a spring
A straight line from the origin up to the elastic limit of the spring.
(the spring undergoes elastic deformation)
Beyond the elastic limit, the spring undergoes plastic deformation.
(and the spring will not return to its original shape when the force is removed)
What is the force constant?
A measure of the stiffness of a spring
(the larger the force constant, the stiffer the spring)
What are the steps to investigation Hooke’s law?
- Attach the spring at one end using a clamp, boss, and clamp stand secured to the bench using a G-clamp or a large mass
- Set up a metre rule with a resolution of 1mm close to the spring
- Suspend slotted masses from the spring and, as you add each one, record the total mass added and the new length of the spring
How can you improve the accuracy when investigation Hooke’s law?
- Take readings at eye level
- Set square for length measurements
- Measure masses using a digital balance
How can you ensure reliable results when investigating Hooke’s law?
Take at least 6 different readings and repeat each one atleast once
What is the equation for elastic potential energy?
E = 1/2 Fx
What cannot be fully recovered if a material has been extended or compressed beyond its elastic limit?
Work done
Draw the force-extension graph of a metal wire
y-axis = Force / N
x-axis = extension / m
straight line through origin (loading graph)
elastic limit and curve
dotted unloading curve parallel to loading curve
Draw the force-extension graph of rubber
y-axis = force / N
x-axis = extension / m
not s shape
starts and ends at origin
arrows for loading and unloading
inside = hysteresis loop
Draw the force-extension graph for polythene
y-axis = force / N
x-axis = extension / m
not s shape for loading graph
straight line down (slight angle) for unloading graph
start at origin
What does the area inside a hysteresis loop represent?
Thermal energy released
What does the extension of a wire depend on>
- The original length of the wire
- The diameter of the wire
- The tension in the wire
- The material of the wire
Define Tensile stress
The force applied per unit cross-sectional area of the wire
What is the equation for Tensile stress?
sigma = F / A
sigma = Tensile stress / Pa
F = Force / N
A = Cross-sectional area / m2
Define Tensile strain
The fractional change in the original length of the wire
What is the equation for Tensile strain?
e (3) = x / L
e(3) = Tensile strain
x = Extension / m
L = Original length / M
Draw the stress-strain graph for a metal wire
Define Ultimate tensile strength
The maximum stress that a material can withstand when being stretched before it breaks
A high ultimate tensile strength means the material is…
Strong
How does Young modulus relate to stiffness?
A material with a large Young modulus is stiffer than a material with a smaller one
How do you determine the Young Modulus of a wire?
- Wire clamped securely at one end, passed over a pulley, and slotted with masses at the other end
- Measure length (using ruler), diameter (using micrometer)
- Cross-sectional area = πd2 / 4
- Force = mg
- After each mass, calculate extension
- Calculate stress-strain values and plot graph
- Young modulus = gradient of graph
Wear eye protection in case wire breaks
Measure diameter from several places along wire for accuracy
Take multiple readings for atleast 6 different masses for accuracy
