Core Practical: Investigating Motion

• The Speed Equation: The fundamental mathematical relationship used is $v = \frac{d}{t}$, where $v$ is average speed in $m/s$, $d$ is distance in $m$, and $t$ is time in $s$. This assumes the motion is analyzed over a discrete interval.

• Gravity and Air Resistance: For falling objects, the motion is governed by the downward force of gravity and the upward force of air resistance. While the practical measures average speed, the object may actually be accelerating or reaching a terminal velocity depending on its aerodynamic properties.

• Statistical Reliability: By repeating measurements at the same distance multiple times, the impact of random errors (like slight variations in release timing) is minimized. Calculating a mean time provides a more representative value for the analysis.

Manual vs. Electronic Timing

• Manual Timing: Relies on a human operator with a stopwatch. It is prone to reaction time errors (typically $\approx 0.25s$) which become more significant over short distances and high speeds.
• Electronic Timing (Light Gates): Uses infrared beams that start and stop a timer automatically when broken by the object. This eliminates human reaction time and provides much higher precision ($0.001s$ resolution).

• Unit Consistency: Always check that distance is in metres and time is in seconds before calculating speed. If the graph or table uses $cm$ or $ms$, convert them to standard SI units ($m$ and $s$) to ensure the final speed is in $m/s$.

• Graphing Skills: When plotting distance against time, the gradient of the line represents the speed. A straight line through the origin indicates constant speed, while a curve indicates acceleration.

• Identifying Anomalies: In exam questions, look for time readings that are significantly different from others in a set of repeats. These should be excluded from the average calculation to maintain accuracy.

• Significant Figures: Ensure your calculated speed is rounded to an appropriate number of significant figures, usually matching the least precise measurement taken during the experiment.

• Parallax Error: This occurs when the scale of the ruler is read from an angle rather than at eye level. This leads to consistent over- or under-estimation of the starting height.

• Reaction Time Neglect: Students often forget that human reaction time is a systematic error in manual timing. To reduce its impact, one should increase the distance of the fall so that the reaction time is a smaller percentage of the total measured time.

• Zero Errors: Failing to ensure the stopwatch is reset to zero or that the ruler starts exactly at the point of release can introduce constant errors into every data point.