IGCSE Physics 0625 - 1.2 Motion

Objective: Recal and use the equation average speed = total distance travelled / total time taken

This section focuses on understanding and applying the concept of average speed. Average speed is a fundamental concept in kinematics, describing how quickly an object is moving over a given distance. It's crucial to distinguish average speed from instantaneous speed, which is the speed at a specific point in time.

Definition of Average Speed

Average speed is defined as the total distance travelled divided by the total time taken for the journey.

Equation

The equation for average speed is:

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

Units

The standard unit for speed in the SI system is meters per second (m/s). Other common units include kilometers per hour (km/h) and miles per hour (mph). It's important to be able to convert between these units.

Example Calculations

Let's work through some examples to illustrate how to use the average speed equation.

Example 1:

A car travels a distance of 240 km in 4 hours. What is its average speed?

Quantity	Value	Unit
Total Distance	240 km	km
Total Time	4 hours	h

Average Speed = $\frac{240 \text{ km}}{4 \text{ h}} = 60 \text{ km/h}$

Example 2:

A runner completes a 100-meter race in 10 seconds. What is their average speed?

Quantity	Value	Unit
Total Distance	100 m	m
Total Time	10 s	s

Average Speed = $\frac{100 \text{ m}}{10 \text{ s}} = 10 \text{ m/s}$

Important Considerations

• Average speed does not account for changes in speed during a journey.
• It's important to use consistent units for distance and time.
• When converting units, ensure the conversion factor is applied correctly.