Paper Rocket Shootout: A force is simply a push or pull (usually on an object). Forces are needed to change the motion of objects. In this activity, the force to push the paper rocket that you construct is provided when you blow air into the straw. The air from your lungs travels through the straw and pushes on the rocket propelling it into space. The harder you push on an object, the more its motion will change. In this case, the harder you blow on the rocket, the further and/or faster it will travel. Key Concepts: motion, forces, pushes, pulls.