touch screen sides to control

// in show() method
Gdx.input.setInputProcessor(new InputAdapter() {

    @Override
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        if(screenX < Gdx.graphics.getWidth() / 4) // adjust the number to divide with
            movement.x = -speed;
        else if(screenX > Gdx.graphics.getWidth() - Gdx.graphics.getWidth() / 4)
            movement.x = speed;
        if(screenY < Gdx.graphics.getWidth() / 4) // y axis is inverted because it's in screen coordinates
            movement.y = speed;
        return true;
    }

    @Override
    public boolean touchUp (int screenX, int screenY, int pointer, int button) {
        movement.set(0, 0);
        return true;
    }

});

// in render(float) method
ball.applyForceToCenter(movement, true);

camera.position.set(ball.getPosition().x, 1.5F, 0);
camera.update();

debugRenderer.render(world, camera.combined);
}