Make the arrow keys work too.

sfmldemo/main.cpp

@@ -126,13 +126,11 @@ int main() {
           break;
         case sf::Event::KeyPressed:
           if(sf::Keyboard::isKeyPressed(sf::Keyboard::Left)) {
-            shape.move(-20, 0);
+            b2Vec2 force(-200, 1000);
+            box.body->ApplyForceToCenter(force, true);
           } else if(sf::Keyboard::isKeyPressed(sf::Keyboard::Right)) {
-            shape.move(20, 0);
-          } else if(sf::Keyboard::isKeyPressed(sf::Keyboard::Up)) {
-            shape.move(0, -20);
-          } else if(sf::Keyboard::isKeyPressed(sf::Keyboard::Down)) {
-            shape.move(0, 20);
+            b2Vec2 force(200, 1000);
+            box.body->ApplyForceToCenter(force, true);
           }
           break;
         case sf::Event::MouseButtonPressed: