Speed (Cambridge (CIE) IGCSE Combined Science: Physics): Revision Note

Speed

• The speed of an object is defined as

  Distance travelled per unit time

• Speed is a scalar quantity

  • This is because it only contains a magnitude (without a direction)

• For objects that are moving at a constant speed, the equation for calculating speed is:

$v=\frac{s}{t}$

• Where:

  • $v$ = speed, measured in metres per second (m/s)

  • $s$ = distance travelled, measured in metres (m)

  • $t$ = time, measured in seconds (s)

Average speed

• The speed of an object can vary throughout its journey

• Therefore, it is often more useful to know an object's average speed

Examples of average speeds

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 an object is:

 $\mathrm{Average}\mathrm{speed}=\frac{\mathrm{distance}\mathrm{travelled}}{\mathrm{time}\mathrm{taken}}$

• Average speed considers the total distance travelled and the total time taken

Formula triangle for average speed, distance moved and time taken

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 average speed, cover-up the variable speed so that only distance and time are left

  • The equation is revealed as:

$\mathrm{speed}=\frac{\mathrm{distance}}{\mathrm{time}}$

To use a formula triangle, simply cover up the quantity you wish calculate and the structure of the equation is revealed