# 5.6.4 Calculating Speed

### Calculating Speed

• For objects that are moving with a constant speed, use the equation below to calculate the speed:  • Where:
• v = speed in metres per second (m/s)
• s = distance travelled in metres (m)
• t = time taken in seconds (s)

#### Worked Example

Planes fly at typical speeds of around 250 m/s.

Calculate the distance travelled by a plane moving at this speed for 2 hours.

Step 1: List the known quantities

• Speed, v = 250 m/s
• Time, t = 2 hours

Step 2: Write the relevant equation Step 3: Rearrange for distance travelled, s

s = v × t

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 in the values for speed and time

s = 250 × 7200 = 1 800 000 m

### Calculating Average Speed

• In some cases, the speed of a moving object is not constant
• For example, the object might be moving faster or slower at certain moments in time (accelerating and decelerating)
• Because its speed is not constant, it is moving with non-uniform motion
• The equation for calculating the average speed for non-uniform motion is: • This can be rearranged with the help of a formula triangle as shown: Average speed, total distance, time formula triangle

#### Worked Example

Florence Griffith Joyner set the women’s 100 m world record in 1988, with a time of 10.49 s.

Calculate her average speed during the race.

Step 1: List the known quantities

• Distance, s = 100 m
• Time, t = 10.49 s

Step 2: Write the relevant equation

• Sprinters typically speed up out of the blocks up to some maximum speed
• Because Florence’s speed changes over the race, we can calculate her average speed using the equation:

average speed = total distance ÷ time taken

Step 3: Check any unit conversions

• Check that all quantities given in the question are in standard units
• In this example, they are all in standard units

Step 4: Substitute the values for total distance and time

Average speed = 100 ÷ 10.49 = 9.53288… = 9.53 m/s ### Author: Jonathan

Jonathan graduated with a first-class Master's degree in Theoretical Physics from Imperial College London. He has worked in education for more than a decade as a Maths and Physics Teacher, Tutor, Head of Physics, and most recently, as Assistant Headteacher. He is now an Educational Consultant and works with us to design and improve our Physics resources.
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