# Scratch Advanced – Week 13 – Gravity and Speed

What are we going to do this week?

• Gravity
• Speed
• Momentum

We are going to look at Gravity and how it works here on Earth. We will also be looking at how, when you travel fast enough, you can appear to defy Gravity, this is how Satellites stay in orbit around the Earth.

We will also take a look at momentum and how things behave once they are moving.

And in order to demonstrate the effects of Gravity,  we will build a Spaceship Game, were you have to launch yourself into Orbit.

Depending upon how we get on this may run over two weeks.

Now back to Gravity. If you  want to get all smart, there is a very clever equation that Sir Issac Newton came up with back in 1687. But we won’t go there for now.

Gravity is basically a force of attraction and the closer you get to something the stronger that force is, also the bigger somehting is the more gravitational force it will have.

It’s a very small force, it takes the whole of the planet Earth to hold us down.

So how are we going to show this in Scrartch, well we will start with a Rocket on the Earth, that we have to Launch upwards, Rockets produce Thrust, which pushes them up into the air. The Earth has Gravity which is trying to pull the rocket back down, but the further you travel away from the Earth the weaker the Gravity and so the Less Thrust you need.

I think I just spotted three Variables there…

1. Thrust

2. Gravity

3. Height

They might come in useful in our program.

We are going to try and build a Scratch Game that will launch a rocket into orbit. This mioght take us a couple of weeks, but should be good fun.

So lets try and write down the rules that we need the Game to obey:

1. Pushing the up arrow should fire the rocket and get us moving.

2. If we stop firing the rocket then gravity will start to pull us back down

3. The higher we go the less Gravity can pull us back down.

4. If we travel fast enough, it can make us appear to defy Gravity, that’s how satellites stay in orbit, and the Moon for that matter. But we will leave this out to start off with as it can start to get really complicated 🙂

To start us off I found a Lunar Lander Game that has some of the elements that we need, this will be a good starting point to  get us going.

To start off we only need a simple sprite, a ball with two legs, that should do for our rocket 🙂