Trying to understand box2d

BaseScreen.java

package com.mygdx.game.screen;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.InputMultiplexer;
import com.badlogic.gdx.InputProcessor;
import com.badlogic.gdx.Screen;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.scenes.scene2d.Stage;
import com.badlogic.gdx.utils.viewport.FitViewport;
import com.badlogic.gdx.utils.viewport.Viewport;

public class BaseScreen implements Screen, InputProcessor {
    protected Viewport viewport;
    protected Stage stage;

    @Override
    public boolean keyDown(int keycode) {
        return false;
    }

    @Override
    public boolean keyUp(int keycode) {
        return false;
    }

    @Override
    public boolean keyTyped(char character) {
        return false;
    }

    @Override
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        return false;
    }

    @Override
    public boolean touchUp(int screenX, int screenY, int pointer, int button) {
        return false;
    }

    @Override
    public boolean touchDragged(int screenX, int screenY, int pointer) {
        return false;
    }

    @Override
    public boolean mouseMoved(int screenX, int screenY) {
        return false;
    }

    @Override
    public boolean scrolled(int amount) {
        return false;
    }

    @Override
    public void show() {
        viewport = new FitViewport(1280, 720);
        stage = new Stage(viewport);

        Gdx.input.setInputProcessor(new InputMultiplexer(this, stage));
    }

    @Override
    public void render(float delta) {
        Gdx.gl.glClearColor(0, 0, 0, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        stage.draw();
    }

    @Override
    public void resize(int width, int height) {
        stage.getViewport().update(width, height, true);
    }

    @Override
    public void pause() {

    }

    @Override
    public void resume() {

    }

    @Override
    public void hide() {

    }

    @Override
    public void dispose() {

    }
}

GameScreen.java

package com.mygdx.game.screen;

import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.*;
import com.mygdx.game.MyGdxGame;

public class GameScreen extends BaseScreen {

    Image ballImage;
    World world;
    Box2DDebugRenderer debugRenderer = new Box2DDebugRenderer();
    Body ballBody;

    @Override
    public void show() {
        super.show();
        Box2D.init();
        ballImage = new BallImage(MyGdxGame.assets.get("PNG/Balls/Blue/ballBlue_02.png", Texture.class));
        stage.addActor(ballImage);

        world = new World(new Vector2(0, -10), true);

        createBallBody();

        createGround();

        ballImage.setPosition(ballBody.getPosition().x, ballBody.getPosition().y);
    }

    private void createBallBody() {
        // First we create a body definition
        BodyDef bodyDef = new BodyDef();
        // We set our body to dynamic, for something like ground which doesn't move we would set it to StaticBody
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        // Set our body's starting position in the world
        bodyDef.position.set(100, 500);

        // Create our body in the world using our body definition
        ballBody= world.createBody(bodyDef);

        // Create a circle shape and set its radius to 6
        CircleShape circle = new CircleShape();
        circle.setRadius(/*0.25f */ ballImage.getWidth()/ 2);

        // Create a fixture definition to apply our shape to
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = circle;
        fixtureDef.density = 0.5f;
        fixtureDef.friction = 0.4f;
        fixtureDef.restitution = 0.6f; // Make it bounce a little bit

        // Create our fixture and attach it to the body
        ballBody.createFixture(fixtureDef);

        // Remember to dispose of any shapes after you're done with them!
        // BodyDef and FixtureDef don't need disposing, but shapes do.
        circle.dispose();
    }

    private void createGround() {
        // Create our body definition
        BodyDef groundBodyDef =new BodyDef();
// Set its world position
        groundBodyDef.position.set(new Vector2(0, 0.1f));

// Create a body from the defintion and add it to the world
        Body groundBody = world.createBody(groundBodyDef);

// Create a polygon shape
        PolygonShape groundBox = new PolygonShape();
// Set the polygon shape as a box which is twice the size of our view port and 20 high
// (setAsBox takes half-width and half-height as arguments)
        groundBox.setAsBox(stage.getCamera().viewportWidth, 0.1f);
// Create a fixture from our polygon shape and add it to our ground body
        groundBody.createFixture(groundBox, 0.0f);
// Clean up after ourselves
        groundBox.dispose();
    }

    @Override
    public void render(float delta) {
        super.render(delta);

        debugRenderer.render(world, stage.getCamera().combined);

        world.step(1/45f, 6, 2);

        ballImage.setPosition(ballBody.getPosition().x, ballBody.getPosition().y);
    }

}