Forces of Flight - Weight

 People can't fly. If we jump up in the air, gravity pulls us back down.  Our weight is caused by the pull of the Earth's gravity. Weight is the force with which gravity presses us against the ground. (We feel our weight in the reaction force of the ground pushing up against our feet).  How can a balloon help us get off the ground?

 A balloon can lift a person off the ground. But the balloon must be less dense than air. (The balloon must be filled with a gas that is lighter than the same volume of air).  Fill the balloon with hot air or helium. Then you can make it float upwards.  How can you float?

 You can float in water. The water around you pushes back on you to balance the force of your weight.  You float because you are less dense than water. (You are lighter than the same volume of water).  An aircraft is much denser than air. So what force keeps it up?

 An aircraft can't float in the air like a balloon. It needs an extra force to keep it up.  Its wings are tilted and specially designed to give the extra force. The force is called lift.    Try a related activity...