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Speed (Edexcel GCSE Physics: Combined Science)
Revision Note
Calculating 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)
- For objects that are moving with a constant speed, use the equation below to calculate the speed:
- Where:
- Speed is measured in metres per second (m/s)
- Distance travelled is measured in metres (m)
- Time taken is measured in seconds (s)
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
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)
- The equation for calculating the average speed of an object is:
- The formula for average speed (and the formula for 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)
How to use formula triangles
Worked example
Planes fly at typical speeds of around 250 m/s.Calculate the total 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 total distance
total distance = 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
total distance = 250 × 7200 = 1 800 000 m
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