Speed.

• if you go 30 miles in 1 hour, your speed is 30 miles an hour
• if you travel 2 metres in 1 second, your speed is 2 metres a second

Acceleration

If you go faster and faster we call it "accelerating".
For example: when something falls to the ground it goes faster and faster.
Until it hits the floor and stops!

When something goes slower and slower it is sometimes called "decelerating".
For example: when you throw a ball up into the air it goes slower and slower, at first.
It's decelerating.

Then, it gets as high as it is going to go.
After that it falls down, going faster and faster.
Now it's accelerating.

Gravity

Gravity is the acceleration everything has when it falls.
Well, I have made things a bit too simple.
If you drop a heavy ball and a feather from the same height, the ball will get to the ground first.
This is why:

The ball and the feather are both slowed down by the air, as they fall through it.
The ball is slowed down a little bit.
The feather is slowed down a lot.
So that is why the ball gets to the ground first.

But if they were dropped in a place where there was no air, they would reach the ground at the same time.
This is why I said that the acceleration we call 'Gravity' is the same for everything.

If you want to watch the scientist Brian Cox doing an experiment where he drops a ball and some feathers where there is no air to slow them down, click here:

We see, at 3 minutes and 19 seconds into the video, that the ball and the feathers get to the ground at the same time!
Don't worry about the rest of the video. Brian Cox talks about a very complicated bit of science, which you can learn when you are older!

So, in our games, we will have a number called 'Gravity' which we will use to make everything fall faster and faster.
Then, they will look just the way they would in real life.