Topic 15 – Forces and matter — Edexcel GCSE Physics

Topic 15 explores how forces affect the dimensions and internal states of physical materials. Students investigate the relationship between tensile force and extension, identifying the linear region governed by Hooke's Law and the non-linear behaviour that occurs past the limit of proportionality.

For Higher Tier candidates, this topic extends into the mechanics of fluids, exploring how pressure varies with depth and density, why atmospheric pressure decreases with altitude, and how differences in fluid pressure give rise to upthrust and floating.

• →Hooke's Law states that force is directly proportional to extension provided the limit of proportionality is not exceeded (F = k * x).
• →Work done in stretching or compressing a material within its limit of proportionality equals the elastic potential energy stored: E = 0.5 * k * x^2.
• →Pressure in a fluid increases with depth and density according to P = h * rho * g, acting equally in all directions.
• →Upthrust is an upward force exerted on a submerged or partially submerged object equal to the weight of the fluid displaced (Archimedes' principle).

• Records the original length of the spring before any load is added and measures each new length with a ruler held parallel to the spring.
• Calculates the extension as the new length minus the original length rather than using the total length as the extension.
• Converts each hanging mass to a force using W = m × g with g = 9.8 N/kg and states the unit of force as newtons.
• Plots force on the vertical axis against extension on the horizontal axis and identifies the straight-line region through the origin as Hooke's law behaviour.
• Calculates the spring constant from the gradient of the straight-line section and gives the unit as N/m.
• Finds the work done from the area under the force–extension graph and uses W = ½ × F × x for the straight-line region.
• Describes control variables such as using the same spring and adding masses one at a time, and explains how repeats or a fiducial marker reduce uncertainty.

• 💡State clearly which quantity is the independent variable and which is dependent before describing the graph.
• 💡Show the equation and substitution for at least one calculation of force, spring constant or work done.
• 💡Explain how you identified the limit of proportionality from the shape of the graph rather than asserting it.
• 💡Link your conclusion back to Hooke's law and to the energy stored in the spring.
• 💡When plotting force-extension graphs from Core Practical data, always plot force on the y-axis and extension on the x-axis to find the spring constant directly from the gradient.
• 💡In multi-step pressure questions, always identify the specific cross-sectional area in contact with the surface and verify that units are in metres squared before dividing.
• 💡For 6-mark questions describing the core practical with springs, mention measuring original length with a metre rule at eye level, using a pointer/fiducial marker to avoid parallax error, and calculating extension progressively.

• Uses the total length of the spring as the extension; subtract the original length to obtain the extension.
• Plots mass on the graph instead of force; convert mass to weight with W = m × g before plotting.
• Treats the whole graph as a straight line through the limit of proportionality; identify where the line curves and restrict the gradient calculation to the straight region.
• Omits the factor of one half when calculating stored energy; use W = ½ × F × x for the linear region.
• Believing that extension is the total length of a stretched spring rather than the change in length (extended length minus original length).
• Thinking that atmospheric pressure increases with altitude; in reality, air density and the weight of the air column above both decrease with height, reducing pressure.
• Assuming an object floats because it is 'light'; floating is determined by average density compared to the fluid and whether upthrust equals weight.

1. 1Step 1: Master Hooke's Law definitions, elastic vs inelastic deformation, and complete five practice calculations using F = k * x.
2. 2Step 2: Review the Core Practical for investigating force and extension; practise sketching and interpreting linear and non-linear force-extension graphs.
3. 3Step 3: Practise energy calculations using E = 0.5 * k * x^2 and determine energy stored from the area under a force-extension graph.
4. 4Step 4 (Higher Tier): Study fluid pressure (P = h * rho * g), atmospheric pressure changes with height, and solve upthrust/floating problems.
5. 5Step 5: Complete a timed set of Edexcel past paper exam questions combining both spring mechanics and fluid pressure.

• 📋Core Practical method questions: Explaining how to measure the extension of a spring reliably using clamps, masses, fiducial markers, and mm-scale rules.
• 📋Graphical analysis questions: Determining the spring constant from the gradient of a straight-line force-extension graph or calculating work done from the area under the curve.
• 📋Higher Tier multi-step fluid calculations: Using depth, fluid density, and gravitational field strength to calculate pressure differences and resultant upthrust forces.