3.4 Materials Flashcards
How do you combine force constant for springs in series?
1/k=1/k1 + 1/k2
How do you combine force constant for springs in parallel?
k=k1 + k2
A material will show elastic deformation up to its … and ……beyond it
A material will show elastic deformation up to its ELASTIC LIMIT and PLASTIC DEFORMATION beyond it
Explain how a material returns to its original shape
- When the material is put under tension, the atoms of the material are pulled part from one another.
- Atoms can move slightly relative to their equilibrium positions, without changing position in the material
- Once the load is removed, the atoms return to their equilibrium distance apart
Explain how a material is permanently stretched
- 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
write up an experiment to find spring constant
- Set up equipment shown in the diagram. Support the object being tested at the top with a clamp and measure its original length with a ruler
- Add weight one at a time to the bottom of the object
- After each weight is added, measure the new length of the object, then calculate the extension.
Extension = new length - original length - Plot a graph of force (weight) against extension. Line of best fit is straight =, then the object obeys hookes law, the gradient =K.
The graph curves if you’ve loaded the object beyond its limit of proportionality - safety: Stand up so you can get out of the way if weight falls quick, safety goggles to protect eyes in case object snaps
What is extension and compression also known as?
Tensile deformation and compresive deformation
What is ultimate tensile strength?
Maximum stress a material can take
What is a compressive force?
Forces acting towards the centre of the spring in both directions. The spring undergoes compressive deformation as a result and will be
shortened.
What is a Tensile force?
Tensile forces act away from the centre of the spring in both directions, and will stretch it out.
What is Hooke’s law?
A material within its elastic limit, the force applied is directly proportional to the extension of the material.
What could you say K (spirng constant) is a measure of?
Stiffness
elastic deformation
it will return to its original shape when the force is removed.
plastic deformation
meaning that when the force is removed, the spring will experience permanent deformation and will not return to its original length
What does the F-e graph look like for a wire?
Knife
Loading, then hits elastic limit curve and then unloading straight line below, this shows permanent extension
What does the F-e graph look like for Rubber?
Rubber is does not experience plastic deformation, but it does NOT obey Hooke’s law. The area between the loading and unloading curves is a hysteresis loop. This area represents the energy that was required to stretch the material out, which was transferred to thermal energy when the force was removed. (Looks like s slanted when loading)
What does the F-e graph look like for Polyethene?
Polyethene is a polymeric material. It does NOT obey Hooke’s law, and experiences plastic deformation when any force is applied to it. This makes it very easy to stretch in to new shapes.
(Looks like slanted s shape when loading and when unloading looks like vertical line slanting towards left)
Area under force extension graph?
Work done
What happens to work done in plastic deformation?
If plastic deformation occurs, then the work done to achieve this deformation is not stored as
elastic potential energy, it is used to rearrange the atoms in to their new permanent positions
Two wires of same material but different lengths and areas are joined end to end. What do they have the same of? (negligible weight)
Tensile force
brittle
Doesn’t undergo plastic deformation, and breaks after elastic limit exceeded
Malleable
Can be drawn into wires
