# Edexcel IGCSE Physics: Double Science 复习笔记：1.1.2 Speed

### Calculating Average Speed

• The speed of an object is the distance it travels every second
• Speed is a scalar quantity
• This is because it only contains a magnitude (without a direction)

A hiker might have an average speed of 2.0 m/s, whereas a particularly excited bumble bee can have average speeds of up to 4.5 m/s

• The equation for calculating the average speed of a moving object is:

• The formula for average speed can be rearranged with the help of the formula triangle below:

#### How to Use Formula Triangles

• Formula triangles are really useful for knowing how to rearrange physics equations
• To use them:

1. Cover up the quantity to be calculated, this is known as the 'subject' of the equation
2. Look at the position of the other two quantities
• If they are on the same line, this means they are multiplied
• If one quantity is above the other, this means they are divided - make sure to keep the order of which is on the top and bottom of the fraction!

• In the example below, to calculate speed, cover-up 'speed' and only distance and time are left
• This means it is equal to distance (on the top) ÷ time (on the bottom)

#### Worked Example

Planes fly at typical speeds of around 250 m/s.Calculate the distance travelled by a plane moving at this average speed for 2 hours.

Step 1: List the known quantities

• Average speed = 250 m/s
• Time taken = 2 hours

Step 2: Write the relevant equation

Step 3: Rearrange for the distance moved

distance moved = average speed × time taken

Step 4: Convert any units

• The time given in the question is not in standard units
• Convert 2 hours into seconds:

2 hours = 2 × 60 × 60 = 7200 s

Step 5: Substitute the values for average speed and time taken

distance moved = 250 × 7200 = 1 800 000 m