Topic 2: Mechanics — Edexcel A-Level Physics
Test yourself on Topic 2: Mechanics with PEARSON EDEXCEL A-Level practice questions.
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Topic 2: Mechanics explained
Graphs encode motion through gradients and areas.
Read the full explanation
On a displacement–time graph the gradient gives velocity; on a velocity–time graph the gradient gives acceleration and the area between the line and the time axis gives displacement. On an acceleration–time graph the area gives change in velocity. For non-uniform acceleration the gradient of a velocity–time curve changes continuously, so you find instantaneous acceleration by drawing a tangent at a point, and displacement by estimating the area under the curve, for example by counting squares or splitting it into trapezium strips. Signs matter: area below the time axis counts as negative displacement or negative velocity change. Always state the quantity, its unit and its sign, and check that the axis units are converted consistently before calculating.
Your focus
- Identify the physical quantity obtained from the gradient of displacement–time, velocity–time and acceleration–time graphs.
- Identify the physical quantity obtained from the area under velocity–time and acceleration–time graphs.
- Apply tangent-drawing and area-estimation methods to a velocity–time curve representing non-uniform acceleration.
Topic 2: Mechanics exam tips
Marking Points
- Gradient of a displacement–time graph equals velocity, with unit m s⁻¹.
- Gradient of a velocity–time graph equals acceleration, with unit m s⁻².
- Area under a velocity–time graph equals displacement (or change in displacement), with unit m.
- Area under an acceleration–time graph equals change in velocity, with unit m s⁻¹.
- For non-uniform acceleration, instantaneous acceleration is found from the tangent to the velocity–time curve at that instant.
- Displacement for a curved velocity–time graph is found by estimating the area, e.g. counting squares or trapezium strips, and areas below the axis are negative.
Examiner Tips
- 💡Write the quantity, its unit and its sign for every gradient or area you quote.
- 💡Convert axis units (for example cm to m, or minutes to seconds) before calculating a gradient or area.
- 💡When a curve is involved, say explicitly that you are drawing a tangent or estimating area, and show your method.
Common Mistakes
- Confusing which graph gives which quantity: the correction is to link gradient to the rate of change of the vertical quantity and area to the product of the vertical quantity and time.
- Treating the area under a displacement–time graph as displacement: the correction is that area under a displacement–time graph has no standard physical meaning here; displacement is read from the vertical axis.
- Ignoring the sign of areas below the time axis: the correction is to subtract such areas because they represent negative displacement or negative change in velocity.
- Using the chord between two points instead of a tangent when asked for instantaneous acceleration on a curve: the correction is to draw a tangent at the single point.