Speed

Definition of Speed

Speed is a scalar quantity, meaning it has magnitude only and no direction. It tells us how fast an object is moving, but not the direction of motion.

It is defined as the distance travelled per unit time. The common units for speed are metres per second (m/s) and kilometres per hour (km/h).

For example, if a car travels 100 metres in 5 seconds, its speed is the distance divided by the time:

$\text{Speed} = \frac{\text{Distance}}{\text{Time}} = \frac{100 \text{ m}}{5 \text{ s}} = 20 \text{ m/s}$

Calculating Speed

The formula to calculate speed is:

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

It is important to use consistent units when calculating speed. Distances should be in metres (m) or kilometres (km), and time in seconds (s) or hours (h). If necessary, convert units before calculating.

For example, if a cyclist travels 15 kilometres in 30 minutes, first convert 30 minutes to hours:

$30 \text{ minutes} = \frac{30}{60} = 0.5 \text{ hours}$

Then calculate speed:

$\text{Speed} = \frac{15 \text{ km}}{0.5 \text{ h}} = 30 \text{ km/h}$

Measuring Speed

To measure speed practically, you need to measure the distance travelled and the time taken accurately.

• Use a measuring tape or trundle wheel to measure the distance.
• Use a stopwatch to time how long it takes to cover that distance.

The speed calculated from these measurements is the average speed over that distance.

For example, if a sprinter runs 100 m in 12 seconds, their average speed is:

$\text{Speed} = \frac{100 \text{ m}}{12 \text{ s}} \approx 8.33 \text{ m/s}$

Instantaneous speed is the speed of an object at a specific moment in time, which can vary during the motion. This is different from average speed, which is calculated over a time interval.

Speed in Everyday Contexts

Speed is an important part of everyday life and affects safety and efficiency.

• Walking speed: Typically about 1.4 m/s (5 km/h).
• Running speed: Varies widely; a fit person might run at 3 6 m/s.
• Driving speed: Speed limits on UK roads vary, e.g., 30 mph (48 km/h) in towns, 70 mph (113 km/h) on motorways.

Speed limits are set to keep drivers safe and reduce accidents. Higher speeds increase stopping distances and the severity of crashes.

The effects of speed on motion include how quickly you reach a destination and how much control you have over the vehicle or object.