OCR GCSE Physics A (J249) — Forces
Every time you stretch a bungee cord or a rubber band, you are testing the relationship between the force you apply and how much the material stretches.
If you plot a graph of force applied against the extension of a typical metal spring, you initially get a straight line passing through the origin .
Hooke's Law only applies up to a specific point, known as the .
Once the limit of proportionality is exceeded, the force-extension graph becomes a curved line, indicating a non-linear relationship.
Students often calculate total extension using the incremental distance between individual weights instead of subtracting the original unstretched length from the new total length.
For OCR graph questions, examiners explicitly penalise 'dot-to-dot' lines in the non-linear region; always draw a smooth curve once the limit of proportionality is passed.
When describing Hooke's Law, remember that 'linear' means any straight line, but 'proportional' means a straight line that specifically passes through the origin .
In the exam, clearly mark the limit of proportionality with an 'X' or a distinct dot exactly where the straight line starts to curve, as vague labelling will lose marks.
Extension
The increase in length of an object when a force is applied, calculated as the stretched length minus the original unstretched length.
Linear relationship
A relationship between two variables that produces a straight-line graph, indicating they change at a constant rate.
Hooke's Law
The principle stating that the extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded.
Spring constant
A measure of an object's stiffness, representing the force required per unit of extension.
Elastic deformation
A temporary change in shape where the object returns to its original dimensions when the applied force is removed.
Limit of proportionality
The exact point on a force-extension graph beyond which extension is no longer directly proportional to the force applied.
Elastic limit
The maximum force or extension from which an object will still return to its original shape when the force is removed.
Non-linear relationship
A relationship between variables that produces a curved line on a graph, indicating they do not change in direct proportion.
Plastic deformation
A permanent change in shape that occurs when an object is stretched beyond its elastic limit and cannot return to its original dimensions.
Put your knowledge into practice — try past paper questions for Physics A
Extension
The increase in length of an object when a force is applied, calculated as the stretched length minus the original unstretched length.
Linear relationship
A relationship between two variables that produces a straight-line graph, indicating they change at a constant rate.
Hooke's Law
The principle stating that the extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded.
Spring constant
A measure of an object's stiffness, representing the force required per unit of extension.
Elastic deformation
A temporary change in shape where the object returns to its original dimensions when the applied force is removed.
Limit of proportionality
The exact point on a force-extension graph beyond which extension is no longer directly proportional to the force applied.
Elastic limit
The maximum force or extension from which an object will still return to its original shape when the force is removed.
Non-linear relationship
A relationship between variables that produces a curved line on a graph, indicating they do not change in direct proportion.
Plastic deformation
A permanent change in shape that occurs when an object is stretched beyond its elastic limit and cannot return to its original dimensions.
