By Brian Snelson from Hockley, Essex, England (Speedometer) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Ever wondered why it’s so difficult for people to catch up to buses by running? It’s a matter of speed. A bus goes faster than a person who is running.
But what does this mean and how do we calculate speed? We measure an object’s average speed by looking at the amount of distance it could travel within a given time. The formula we use to calculate average speed is: Average speed= distance travelled ÷ time taken to travel the given distance (where distance is in meters and time in seconds.) 
Thinking outside the box
The unit for average speed is usually m/s, but it can be changed for the practicality of the value. For instance, cars use the unit km/h. This makes it easier to calculate the time taken to reach a destination that is kilometers away at a given speed, because you don’t have to convert between meters and kilometers, and seconds and minutes. Imagine driving at 90km/ h, equivalent to 90 000m/ 3600 s or 25 m/s, and heading to a place that is 50 km away. When calculating the estimated arrival time with the m/s unit, you will get a large value in seconds, which is not practical as more unit conversions will be required for you to figure out the time required in minutes.
The speed calculated from this equation is likely not equal to the speed of the object at a given time, because the speed may fluctuate during the given time duration. Imagine a car on the road. It may change speed due to the amount of traffic and traffic lights, and the average speed calculated from the equation will be below the fastest speed yet higher than the slowest speed. Thus, the equation is here for you to get an idea of the speed of the object in the time duration, and less for getting an accurate value for the speed at a given time.
Are you ready to put your knowledge to the test? Try the following two questions:
1. You are driving near a beach. All of a sudden, you see Justin Bieber: he was able to drive from a stop sign to a palm tree, an estimated 42 m, in 2 seconds. Your friend is a big fan of his and you decide to catch up to him to get his autograph. What should be your minimum average car speed if you would like to succeed in doing so? *Round answer to the nearest whole number.
2. In a game of plants vs. zombies, a pea pod attacks a zombie with peas. A zombie is now standing at the edge of your garden that is closest to the road! It travels 3 meters in 6 seconds and it must be stopped from reaching your door in the game. It must cross your 6 m by 6 m garden before doing so, and your peapod is just by your door, ready to defend your property. The peapod fires a pea at the zombie to defeat it, and it takes 3 seconds for this pea to reach the other end of your garden.
a. Which is faster: the pea or the zombie? (Hint: calculate their speeds and compare them)
b. How long would it take for the pea to hit the zombie if they move at the same time? (Hint: use the average speed equation and rearrange it to calculate the distance travelled. What is the relationship between the distance travelled by the pea and the zombie and the length of the garden?)
The unit for average speed is usually m/s, but it can be changed for the practicality of the value. For instance, cars use the unit km/h. This makes it easier to calculate the time taken to reach a destination that is kilometers away at a given speed, because you don’t have to convert between meters and kilometers, and seconds and minutes. Imagine driving at 90km/ h, equivalent to 90 000m/ 3600 s or 25 m/s, and heading to a place that is 50 km away. When calculating the estimated arrival time with the m/s unit, you will get a large value in seconds, which is not practical as more unit conversions will be required for you to figure out the time required in minutes.
The speed calculated from this equation is likely not equal to the speed of the object at a given time, because the speed may fluctuate during the given time duration. Imagine a car on the road. It may change speed due to the amount of traffic and traffic lights, and the average speed calculated from the equation will be below the fastest speed yet higher than the slowest speed. Thus, the equation is here for you to get an idea of the speed of the object in the time duration, and less for getting an accurate value for the speed at a given time.
Are you ready to put your knowledge to the test? Try the following two questions:
1. You are driving near a beach. All of a sudden, you see Justin Bieber: he was able to drive from a stop sign to a palm tree, an estimated 42 m, in 2 seconds. Your friend is a big fan of his and you decide to catch up to him to get his autograph. What should be your minimum average car speed if you would like to succeed in doing so? *Round answer to the nearest whole number.
2. In a game of plants vs. zombies, a pea pod attacks a zombie with peas. A zombie is now standing at the edge of your garden that is closest to the road! It travels 3 meters in 6 seconds and it must be stopped from reaching your door in the game. It must cross your 6 m by 6 m garden before doing so, and your peapod is just by your door, ready to defend your property. The peapod fires a pea at the zombie to defeat it, and it takes 3 seconds for this pea to reach the other end of your garden.
a. Which is faster: the pea or the zombie? (Hint: calculate their speeds and compare them)
b. How long would it take for the pea to hit the zombie if they move at the same time? (Hint: use the average speed equation and rearrange it to calculate the distance travelled. What is the relationship between the distance travelled by the pea and the zombie and the length of the garden?)
Reference
Average speed. (n.d.). BBC Bitesize. Retrieved August 12, 2015, from http://www.bbc.co.uk/education/guides/z3bqtfr/revision