Chapter revision
Revision summary
Key ideas, equations and common mistakes. Open any topic below for the full explanation.
- Average speed
- Total distance / total elapsed time. Include stops. Unit: m/s.
- Acceleration
- (v - u) / t. Constant throughout a uniform interval; otherwise an average. Unit: m/s2.
- Free fall near Earth
- g is approximately 10 m/s2 downwards. Air resistance is neglected.
| Feature | Distance-time | Speed-time |
|---|---|---|
| Height | Distance in m | Speed in m/s |
| Gradient | Speed | Acceleration |
| Horizontal line | At rest | Constant speed; rest only at zero |
| Area | Does not give distance or displacement | Distance travelled |
Measure free-fall acceleration
Two light gates time the passage of the same short flag. Speed = flag length / blocked duration. For approximately uniform acceleration, assign each speed to the midpoint of its blocked interval, then use the time between those midpoints in a = (v - u) / t. The supplied speeds 1.00 and 2.00 m/s at 0.0100 and 0.1120 s give about 9.8 m/s2 downwards. This is a free-fall determination only if other forces are negligible. Align the flag and gates, use a suitable timer and keep the attached body out of the beam; repeats do not fix a wrong flag length or time interval.
Check your answer
- Identify the axes and the time interval.
- Use changes on both axes for a gradient.
- Split graph areas into rectangles and triangles; triangle area = (1/2) x base x height.
- Give units, and direction when the quantity needs it.
- State assumptions such as uniform acceleration or negligible air resistance.
Review a topic
- Distance, speed and velocity
- Acceleration and slowing down
- Distance-time graphs
- Speed-time graphs
- Free fall
- Plotting motion graphs