Forces and elasticity (AQA GCSE Physics): Revision Notes
Forces and elasticity
What is deformation?
When you apply force to materials, they change shape. This change in shape is called deformation. All types of deformation need more than one force to work.
Understanding deformation is essential for engineers and designers when selecting materials for different applications. The way materials respond to forces determines their suitability for various purposes.
Materials can deform in two ways:
- Elastic deformation - the material goes back to its original shape
- Inelastic deformation - the material stays changed permanently
Types of deformation
Bending
Bending occurs when two forces work in opposite directions - one force acts clockwise, the other acts anticlockwise.
Practical Example: Bending a Ruler
When you place a ruler between your hands and apply pressure from both sides, you're applying two opposing forces. One hand pushes down while the other pushes up, creating the bending effect.
Stretching
Stretching requires two forces of tension where both forces pull away from each other.
Practical Example: Rope Pulling
Imagine two people pulling a rope from opposite ends. Each person applies a tension force away from the rope's centre, causing it to stretch.
Compression
Compression happens when two equal forces push towards each other, squashing the material.
Practical Example: Sponge Compression
When you place a sponge between your palms and press them together, both hands apply equal forces towards the centre, compressing the sponge.
Elastic deformation
This happens when materials return to their original shape after you remove the force.
Key features:
- The change is temporary
- No permanent damage occurs
- Common with rubber bands and springs
Worked Example: Elastic Band
When you stretch an elastic band and let it go, it snaps back to its original size. This demonstrates elastic deformation because:
- The material changes shape under force
- The material returns to its original form when the force is removed
- No permanent damage has occurred
Inelastic deformation
This happens when materials do not return to their original shape after you remove the force.
Key features:
- The change is permanent
- The material has been damaged or changed forever
- Common with plasticine and overstretched springs
Worked Example: Overstretched Spring
If you stretch a spring too far beyond its elastic limit:
- The spring becomes longer even when you stop pulling it
- The coils may be permanently deformed
- The spring cannot return to its original length
This demonstrates inelastic deformation.
Force and extension graphs
When you plot force against extension (how much something stretches), you get important information about material behaviour.
- At first: The line is straight. This means force and extension are directly proportional
- Elastic region: The material will return to its original shape
- Limit of proportionality: The point where the line stops being straight
Critical Concept: Limit of Proportionality
The limit of proportionality is a crucial point on force-extension graphs. Beyond this point, the material may not return to its original shape even if the force is removed. This is where elastic behaviour ends and inelastic behaviour begins.
The straight part of the graph shows elastic behaviour. Once you go past the limit of proportionality, the material may not return to its original shape.
Real-world examples
Understanding the difference between elastic and inelastic deformation helps us choose appropriate materials for different applications.
Materials showing elastic deformation:
- Rubber bands (up to a point)
- Metal springs (within limits)
- Bouncy balls
Materials showing inelastic deformation:
- Plasticine or clay
- Overstretched elastic bands
- Bent metal that stays bent
The key phrase "up to a point" and "within limits" is important - even elastic materials will deform inelastically if you apply too much force beyond their elastic limit.
Key Points to Remember:
- All deformation needs at least two forces working together
- Elastic deformation is reversible - materials bounce back to their original shape
- Inelastic deformation is permanent - materials stay changed forever
- Force-extension graphs show the limit where materials stop behaving elastically
- Understanding elasticity helps us choose the right materials for different jobs