Did you know that speed can mean many different things?  It is not just about going as fast as you can.  It's also about how you start, how you finish, how you stay on course, and how fast you go in any given second.  Speed changes.  The best way to learn about the different kinds of speed is by . . . well, moving.  Let's take a trip.  How would you like to do it?  Race car?  Motorcycle?  Airplane?  What did you say?  You're too young to drive those?  That's fine!  This is make believe.  In that case, I have an idea.  I'll transform you into a cheetah.


You are on the plains.  You spot a herd of gazelle eating.  You hide in the bushes.  This is the beginning of the hunt.  Your speed is zero.  Initial speed means how fast something is moving at the beginning point.  You could just as easily already be moving when we first start measuring.  If we were watching a drop of rain fall through the air, it would already be moving when we start watching it.  In this case, you are still.  You wait until one gazelle ducks its necks for a bite of grass, and ZOOM.  You are off.

Look at that. I see lunch up ahead.

Yathin sk, CC BY-SA 3.0 , via Wikimedia Commons

The gazelle runs.  You chase after it.  It has a head start, but that does not matter.  You are a cheetah.  You speed up until you are running at fifty miles per hour.  The gazelle cannot run that fast.  You start to catch up.  You are still going fifty miles per hour.  Constant speed is when something keeps moving at the same speed.  It does not go faster.  It does not go slower.  And right now, you do not have to change your speed.  You are a cheetah.

Maybe not so afraid of my horns.

Profberger at English Wikipedia, CC BY-SA 3.0 , via Wikimedia Commons

The gazelle starts jumping left and right, trying to get away.  Then you lose sight of it in the brush.  Tired, you come to a stop.  That's right, you stop in the middle of the hunt!  But then another gazelle runs past you.  You shoot after it.  This is it.  You will need to run as fast as your cheetah legs can carry you.  Faster.  Faster.  FASTER.  Pause!  You and the gazelle freeze.  It looks like I pressed pause on a movie.  I stopped you because you reached your top speed.  I want to see how fast you are going.  Instantaneous speed is how fast something is moving in one moment.  At different points in the hunt, your speed was changed with all the stopping, turning, and starting again.  Your speed at any one second will be much different then the speed you were moving during the race from start to finish.  Right now, with the plains rushing by, you were running at sixty miles per hour!  That's as fast as a car on a freeway!

60 miles per hour speed limit! No one is going faster than 12 miles an hour.

Prayitno from Los Angeles, USA, CC BY 2.0 , via Wikimedia Commons

You leap, you grab the gazelle, and you both roll.  You have caught your prey.  After you roll to a stop, let's see how fast you are moving.  Once again, you are at zero.  Final speed means the last speed in a measurement.  Just like initial speed, you could have still been moving when we measured this.  If we were to stop measuring someone when they were speeding from the top to the bottom of a hill, their final speed would be much faster at the bottom of the hill.  The chase is done.  You land on top of the gazelle, open your jaws, and . . .

I turn you back into a person.  The gazelle runs away.  No need for any blood.  This was a great way to look at the different ways to talk about speed.  Now we have all kinds of information about how fast you moved at different times during your hunt.  You had a starting speed, a finishing speed, a speed when you were moving in your fastest moment, and then, a final speed.  Are you mad you did not get to eat that cheetah?  Here, have some Cheetos instead.  Those are kind of the same, right?

References:

Physics 4 Kids.  "Velocity, Speed, and Motion"¦ Oh My!"  Physics 4 Kids, 2009.  <http://www.physics4kids.com/files/motion_velocity.html>