Calculating Speed (AQA GCSE Physics: Combined Science)

• 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)

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

• 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

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