In the IGCSE Physics exam you will often be asked to analyse distance‑time or speed‑time graphs. By looking at the shape of the graph you can tell whether an object is at rest, moving at a constant speed, accelerating or decelerating. Below are the key clues, some everyday analogies, and quick exam tips.
Exam Tip: Always read the question carefully – it may ask you to describe the motion at a specific point or over a time interval.
On a distance‑time graph the line is flat (horizontal). On a speed‑time graph the line sits at 0 m s⁻¹.
⚡ Analogy: Think of a parked car – the odometer doesn’t tick up.
Exam Tip: If the graph shows a flat segment, write “the object is at rest during that interval.” No calculation needed.
On a distance‑time graph the line is a straight line with a non‑zero slope. The slope equals the constant speed:
\$v = \frac{\Delta s}{\Delta t}\$
On a speed‑time graph the line is a horizontal line at a positive value.
🚗 Analogy: A car cruising on a highway at 80 km h⁻¹ – the odometer ticks at a steady rate.
Exam Tip: If the slope is constant, state the speed and note that acceleration is zero.
On a distance‑time graph the curve is concave upward (bends upwards). The slope is increasing.
On a speed‑time graph the line is sloping upward – speed rises with time.
🏎️ Analogy: A roller‑coaster that speeds up as it goes down a hill.
Exam Tip: Look for a curve that bends upward. If the slope increases, the object is accelerating. You may be asked to estimate the acceleration by calculating the change in slope over a time interval.
On a distance‑time graph the curve is concave downward (bends downwards). The slope is decreasing.
On a speed‑time graph the line slopes downward – speed decreases with time.
🛑 Analogy: A bicycle coming to a stop as the rider applies the brakes.
Exam Tip: A downward‑sloping line on a speed‑time graph indicates deceleration. If the line reaches zero, the object has stopped.
| Graph Type | Shape | Motion | Acceleration |
|---|---|---|---|
| Distance‑time | Flat | At rest | 0 |
| Distance‑time | Straight line (non‑zero slope) | Constant speed | 0 |
| Distance‑time | Concave upward | Accelerating | Positive |
| Distance‑time | Concave downward | Decelerating | Negative |
| Speed‑time | Flat at 0 | At rest | 0 |
| Speed‑time | Flat (positive) | Constant speed | 0 |
| Speed‑time | Increasing | Accelerating | Positive |
| Speed‑time | Decreasing | Decelerating | Negative |
Final Exam Tip: When you see a graph, first identify its type (distance‑time or speed‑time). Then check the slope or curvature. Use the table above as a quick mental checklist. Remember to state the motion and the sign of the acceleration in your answer. Good luck! 🚀