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:
- Cover up the quantity to be calculated, this is known as the 'subject' of the equation
- 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
