IGCSE Physics 0625

Topic: 1.2 Motion

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

This section focuses on understanding and calculating average speed. Average speed is a fundamental concept in physics, allowing us to describe how quickly an object is moving over a particular distance.

Understanding Average Speed

Average speed is calculated by dividing the total distance an object travels by the total time taken for that journey. It's important to remember that average speed does not tell us about the instantaneous speed of the object – its speed at any particular moment in time.

Formula

The formula for average speed is:

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

Units

The standard unit for speed in the International System of Units (SI) is meters per second (m/s). Other common units include:

• Kilometers per hour (km/h)
• Miles per hour (mph)

It's crucial to use consistent units for distance and time when calculating average speed.

Examples

Here are a few examples to illustrate how to use the average speed formula:

Example 1

A car travels a distance of 200 kilometers in 4 hours. What is its average speed?

Using the formula: $$ \text{Average Speed} = \frac{200 \text{ km}}{4 \text{ h}} = 50 \text{ km/h} $$

Example 2

A runner completes a 100-meter race in 12 seconds. What is their average speed?

Using the formula: $$ \text{Average Speed} = \frac{100 \text{ m}}{12 \text{ s}} \approx 8.33 \text{ m/s} $$

Table Summary

Quantity	Symbol	Unit
Average Speed	$$ v_{avg} $$	m/s, km/h, mph
Total Distance	$$ d $$	m, km, miles
Total Time	$$ t $$	s, h, hours

Important Considerations

• The formula for average speed assumes the object travels at a constant speed throughout the entire journey.
• If the object changes speed during the journey, the average speed will be the total distance divided by the total time, but it doesn't represent the speed at any specific point in time.