Forces, Movement & Changing Shape
Forces & Motion · 1 question type
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.
The law
- states that, up to the , the of an elastic object (such as a spring) is directly proportional to the stretching it
- Directly proportional means:
- doubling the stretching force doubles the extension
- halving the stretching force halves the extension
- a plot of force against extension is a straight line through the origin within the proportional region
Sketching a force-extension graph that obeys Hooke's law
What comes up: "Sketch a graph to show that a spring obeys Hooke's law. Label both axes with appropriate physical quantities."
Write (three marks): (1) label the y-axis "force" (or "load") and the x-axis "extension" (or length, if the line is drawn starting above the origin); (2) draw a straight line with a positive gradient; (3) the line must pass through the origin.
Watch out: if you label the bend in the graph as "elastic limit", the mark scheme ignores that label — you must write "limit of proportionality" to show the boundary between the straight and curved regions for any credit. The mark scheme also ignores units on the axes, so you won't lose marks for omitting them, but you must include the quantity names.
Limit of proportionality
- Every elastic material has a , a stretching force beyond which extension no longer keeps step with the force
- Past this limit, the force-extension graph bends away from its straight section: each extra newton produces a larger extra extension than before
- The exact value of the limit depends on the material and on its dimensions
Elastic and inelastic deformation
- Deformation is any change in an object's original shape produced by a force
- Two outcomes are possible when the force is removed:
- , where the object returns to its original shape and size. Examples: a steel spring, a rubber band, most fabrics, a tennis ball after a bounce
- (also called plastic deformation), where the object stays permanently changed even after the force is removed. Examples: a clay tile that has been moulded, a paperclip that has been bent past straightening, a metal drinks can that has been crushed
- Hooke's law is a statement about the elastic region of a material; once a material has been pushed into inelastic deformation, it does not spring back to the origin of the force-extension graph