Forces and elasticity (AQA GCSE Physics Combined Science): Revision Notes
Forces and elasticity
When forces act on materials, they can change their shape. This change in shape is called deformation. Understanding how materials respond to forces is essential for understanding physics and engineering applications.
What happens when materials are deformed?
Materials can be deformed in two ways:
- Elastic deformation - the material goes back to its original shape when the force is removed
- Inelastic deformation - the material stays changed and doesn't return to its original shape
All deformation needs more than one force to work. A single force acting alone cannot deform a material - there must always be at least two forces acting in different directions.
Types of forces that cause deformation
There are three main ways forces can change the shape of materials:
Bending
- Needs two forces working in opposite directions
- One force turns clockwise, the other turns anticlockwise
- Example: bending a ruler with your hands
Practical Example: Bending a Ruler
When you hold a ruler at both ends and bend it:
- Your left hand applies a force downward
- Your right hand applies a force upward
- These opposite forces create the bending deformation
- When you let go, the ruler returns to its straight shape (elastic deformation)
Stretching
- Needs two forces pulling away from each other
- Both forces create tension in the material
- Example: two people pulling opposite ends of a rope
Compression
- Needs two equal forces pushing towards each other
- The material gets squashed or compressed
- Example: squeezing a sponge between your hands
In compression, the forces must be equal and opposite to avoid the material moving sideways. Unequal forces would cause both compression and movement.
Elastic deformation explained
When a material shows elastic deformation, it behaves like a spring or elastic band. The key points are:
- The material returns to its original shape when you stop applying force
- Elastic bands are a perfect example - stretch them and they snap back
- The material has not been permanently damaged
For small forces, the relationship between force and extension is directly proportional. This means if you double the force, you double the extension.
This proportional relationship only applies within the elastic limit. Beyond this limit, the material's behaviour becomes more complex and eventually leads to permanent deformation.
Inelastic deformation explained
When a material shows inelastic deformation, it has been permanently changed:
- The material does not return to its original shape
- Even when you remove the force, the material stays stretched or bent
- The material has been permanently damaged
A spring that has been overstretched is a good example. It stays longer even when no force is applied.
Once a material has undergone inelastic deformation, its internal structure has been altered. This is why materials like metals become weaker after being bent or stretched beyond their elastic limit.
Understanding the force-extension graph
When you plot force against extension on a graph, you can see important patterns in material behaviour:
- At first, the line is straight - this shows elastic behaviour
- There is a point called the limit of proportionality (point P on the graph)
- Beyond this point, the relationship becomes non-linear
- After the limit, the material will show inelastic deformation
The limit of proportionality is a critical point on the force-extension graph. Below this point, Hooke's Law applies and the material behaves elastically. Above this point, the material begins to deform permanently and may eventually break.
Key Points to Remember:
- Elastic materials return to their original shape when the force is removed
- Inelastic materials stay permanently changed after the force is removed
- All deformation requires more than one force acting on the material
- Stretching needs forces pulling apart, compression needs forces pushing together
- The limit of proportionality shows where elastic behaviour ends on a force-extension graph