PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure
Physics 6.3 Distance-time graphs — Calculating Speed from Gradient
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Lesson summary
Physics 6.3 Distance-time graphs — Calculating Speed from Gradient — Alhussan International Academy — Physics (Stages 7–9) notes: Describe the motion of an object from a distance-time graph and calculate speed from the gradient.Distance-time graph = a graph showing the distance a…
- Steady speed
- Realistic motion
- Average speed for a journey
- Dropped ball
- Slowing down
- Speed at one moment
Section 1 of 5Plotting a distance-time graph
- Distance-time graph = a graph showing the distance an object has travelled from its starting point against time.
- Time is on the horizontal axis and distance is on the vertical axis.
- Data
- Measure the distance travelled from the start at regular times, such as every second, and plot the values.
- Steady speed
- If the object moves the same distance each second, the graph is a straight line.
- Axes
- Give each axis a title and unit and use a scale that fills the grid.
Section 2 of 5Reading the shape of the line
| Line on a distance-time graph | Meaning |
|---|---|
| Flat (horizontal) | Stationary: the distance is not changing, so the speed is zero |
| Straight sloping line | Constant speed |
| Steeper straight line | Faster constant speed |
| Curve getting steeper | Speeding up (accelerating) |
| Curve getting less steep | Slowing down |
- Gradient
- The slope of the line tells you the speed.
- Comparing
- Where two lines cross, the objects are at the same place at the same time.
Section 3 of 5A journey told as a graph
- Sections
- A journey graph can be split into sections.
- Flat sections show the person has stopped.
- Sloping straight sections show movement at a steady speed, with different speeds if the slopes differ.
- Realistic motion
- In real journeys speed changes gradually, so a realistic graph has curved lines instead of sudden changes.
Section 4 of 5Calculating speed from the gradient
- Gradient = change in distance ÷ change in time, which equals the speed.
- Method
- Choose two points on a straight section, find the distance moved and the time taken, and divide.
- Units
- Convert time into seconds if the speed is needed in m/s.
- Average speed for a journey
- Divide the total distance by the total time, including stops.
Section 5 of 5Curved lines
- Dropped ball
- Its speed increases as it falls, so the graph is a curve whose slope increases.
- Slowing down
- The curve becomes less and less steep, showing the speed is decreasing.
- Speed at one moment
- Draw a tangent to the curve at that point and calculate its gradient.
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