- Education and Science
Physics for Kids: Learn All About Speed and Velocity
What Is Speed?
Have you ever heard your parents talking about miles per hour or kilometers per hour? These terms refer to how we measure speed. So what exactly is speed? It’s the distance (how far) a thing travels over a period of time. Let’s make this easier to understand.
Imagine you’re watching a snail. Snails move very slowly. How would you tell someone how slowly it moved? Well, you could measure the distance it traveled and how long it took to travel that distance. Let’s pretend our snail moved three feet (each foot is 12 inches) in an hour. We would say its speed is three feet per hour or 3 feet/hour.
Imagine now that you could put a jetpack on the snail’s shell. It can move much faster now. It takes less time to move three feet. As the speed of the snail increases, the time to go three feet decreases. Perhaps the snail can now travel 3 feet/minute.
Imagine the opposite as well. Instead of a jetpack, pretend a weight has been placed on the snail’s shell. Now it’s more difficult for the snail to move. It will take longer for the snail to travel three feet. As the speed of the snail decreases, the time to travel three feet increases. Perhaps it's travel time will become 3 feet in two hours.
As speed increases, the time it takes to travel a certain distance gets shorter.
As speed decreases, the time to travel a certain distance gets longer.
Acceleration and Deceleration
When you’re in a car, it speeds up and slows down. Speeding up is called acceleration and slowing down is called deceleration.
Acceleration – increasing speed
Deceleration – decreasing speed
The definition or meaning of the word velocity is a little more difficult to understand. Velocity is the speed of something in a given direction. Speed tells how fast an object is moving but not where it’s moving. Think of our snail. It moves three feet per hour. If we know the direction our snail is moving, we can specify its velocity. Let’s say it’s going south. Its velocity is three feet per hour south.
Video: Speed and Velocity are different Song (by They Might Be Giants)
When you travel in a car, you aren’t always traveling at the same speed. Sometimes you’re stopped at traffic lights, so you are traveling 0 miles per hour. The car accelerates when the light turns green going from 0 to 30 miles per hour. The car doesn’t instantly go from 0 to 30 miles per hour though. It takes time for the car to increase its speed. When the car comes to a red light, it decelerates. It doesn’t go immediately from 30 miles per hour to 0 miles per hour. It takes time to stop. So, the speed of the car is often changing.
Speed limits vary as well. The speed limit on one street might be 30 miles per hour but 40 miles per hour on another. The average speed tells us how fast we were traveling over the course of our whole trip.
Let’s use our snail to make this easier to understand. Let's say it was able to move 20 inches in a minute with the jetpack on. Then its jetpack was removed. Without the jetpack it was able to move only 2 inches in a minute. So, it moved 22 inches in 2 minutes. Its average speed is how fast it moved that distance. We use something called a mathematical formula to figure out the average speed.
The formula for average speed is distance traveled divided by the time it took to travel. This might be written in math as:
Average speed = distance traveled / time traveled (the / is another type of division sign)
We can plug the numbers for our snail into this formula.
Average speed = distance traveled (22 inches) / time traveled (2 minutes)
22 divided by 2 is 11. So, the snail’s average speed is 11 inches per foot.