Forces & ElasticityEdexcel GCSE Physics: Revision notes
Section 1
What is elastic and inelastic distortion?
When a force is applied to an object it can stretch, compress or bend it — this is called distortion.
- Elastic distortion: the object returns to its original shape and length once the force is removed
- Inelastic distortion: the object does not return to its original shape and length — it has been permanently deformed
A spring stretched gently shows elastic distortion. A spring stretched too far shows inelastic distortion because the material has been damaged.
Stretch a rubber band a little and let go — it springs back (elastic). Stretch it until it stays baggy — it has been permanently stretched (inelastic).
Section 2
How does force relate to extension?
For a spring, the extension is directly proportional to the force applied, up to a point.
F = k × e
- F = force applied (N)
- k = spring constant (N/m)
- e = extension (m)
This relationship (Hooke's Law) applies to both stretching (extension) and squashing (compression) of a spring.
As force increases, extension increases in a straight-line (directly proportional) graph — until the limit of proportionality is reached. Beyond this point, the graph curves and extension is no longer proportional to force. The elastic limit is the point beyond which the spring will no longer return to its original length — permanent (inelastic) deformation has occurred.
State clearly that extension = final length − natural (original) length, not just 'the length' — examiners penalise this mix-up.
Section 3
What is the spring constant?
The spring constant, k, measures how stiff a spring is.
- Measured in N/m
- A large spring constant means a stiff spring (large force needed for a given extension)
- A small spring constant means a soft/floppy spring
Rearranging F = k × e gives k = F ÷ e, which can be found from the gradient of a force–extension graph while it is a straight line.
Section 4
Core Practical: investigating extension and work done on a spring
Method:
- Clamp a spring vertically and measure its natural (unstretched) length with a ruler
- Add a known mass (weight) to the spring and measure the new length
- Calculate extension = new length − natural length
- Repeat, adding masses in equal steps, up to the elastic limit
- Plot a graph of force (y-axis) against extension (x-axis)
Analysis: the graph is a straight line through the origin while proportional; the gradient equals the spring constant, k. The area under the graph gives the work done (energy transferred) in stretching the spring.
Students often forget to measure the spring's natural length first — without it, extension cannot be calculated correctly.
Section 5
How is elastic potential energy calculated? (Higher tier)
When a spring is stretched (elastically) within its limit of proportionality, work is done on it and energy is stored as elastic potential energy.
Eₑ = ½ × k × e²
- Eₑ = elastic potential energy (J)
- k = spring constant (N/m)
- e = extension (m)
If a spring is stretched beyond the limit of proportionality, the work done stretching it is greater than the elastic potential energy stored. This is because some of the energy is used to permanently deform the spring (e.g. as heat, or rearranging the internal structure) rather than being stored elastically and fully recoverable.
A spring with k = 40 N/m stretched by 0.2 m stores Eₑ = 0.5 × 40 × 0.2² = 0.8 J.
Must Know
- Elastic distortion: object returns to original shape; inelastic: it does not
- F = k × e applies to both stretching and compression
- Extension is proportional to force only up to the limit of proportionality
- Beyond the elastic limit, the spring is permanently deformed
- Spring constant k is in N/m; gradient of force–extension graph = k
- Eₑ = ½ke² (higher tier); work done beyond the limit of proportionality exceeds Eₑ stored
That's the notes covered.
Carry on to the next subtopic.