Measuring Speed

Definition of Speed

Speed is a scalar quantity, which means it only has magnitude (size) and no direction. It tells us how fast an object is moving.

Speed is defined as the distance travelled per unit time. The formula for speed is:

$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$

Common units for speed are metres per second (m/s) and kilometres per hour (km/h). For example, a car travelling at 60 km/h is moving 60 kilometres every hour.

Measuring Distance

To measure speed, you first need to measure the distance an object travels accurately.

• For short distances, use a metre stick to measure in metres (m).
• For longer distances, a measuring tape is more practical and can measure several metres or more.
• A trundle wheel is useful for measuring longer distances outdoors, such as on a sports field or track. It counts the number of wheel rotations to calculate distance.

Always record the distance in metres (m) to keep units consistent when calculating speed in m/s.

Measuring Time

Next, you need to measure how long it takes for the object to travel the measured distance.

• A stopwatch is the most common tool for measuring time in seconds (s).
• Start the stopwatch exactly when the object begins moving and stop it immediately when it reaches the end of the measured distance.
• To improve accuracy, take multiple timings of the same run and calculate the average time.

Accurate timing is essential because even small errors can significantly affect the calculated speed.

Calculating Speed

Once you have the distance and time, calculate speed using the formula:

$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$

Make sure the distance and time units are compatible. For example, if distance is in metres and time in seconds, speed will be in metres per second (m/s). If units differ, convert them first to compatible units before calculating speed.

To convert speed from m/s to km/h, multiply by 3.6:

$$\text{Speed (km/h)} = \text{Speed (m/s)} \times 3.6$$

For instance, if a cyclist travels 150 metres in 30 seconds, the speed is:

$$\text{Speed} = \frac{150}{30} = 5 \text{ m/s}$$