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If you've ever watched a jet plane taking off or coming in to land, the first thing you'll have noticed is the noise of the engines. Jet engines, which are long metal tubes burning a continuous rush of fuel and air, are far noisier (and far more powerful) than traditional propeller engines. You might think engines are the key to making a plane fly, but you'd be wrong. Things can fly quite happily without engines, as gliders (planes with no engines), paper planes, and indeed gliding birds readily show us.

Have you ever opened the hood of your car and wondered what was going on in there? A car engine can look like a big confusing jumble of metal, tubes and wires to the uninitiated.

Aircraft do not defy the law of gravity, they produce a sufficient force to overcome gravity, just as you do if you decide to jump in the air.

There are four main forces acting on an aircraft, thrust from the engines, drag on the airframe, gravity pulling down and lift generated by the wings.

Once the size of the lift is greater than the weight of the plane then gravity will be overcome and the plane will rise. Once cruising altitude is achieved the lift force is balanced with the weight and the aircraft stays at a constant height.

If you experience engine trouble, you take your car to a mechanic. If your pipes leak, you call a plumber. And if you fracture a leg, the usual course of action is to visit a doctor. But unlike other things that may break in life, bones begin healing on their own before you even set foot in a waiting room.The human body possesses amazing healing powers that enable it to bounce back from a vast array of illnesses and injuries. Sometimes broken bones can heal so thoroughly within a few months that even an x-ray can't determine the original fracture line.Doctors often play a vital, sometimes lifesaving, role in a bone's healing process. But, these experts basically help the body heal itself. Doctors provide optimal conditions for bone repair and healing to take place. The rest is up to your cells.But how does this amazing biological process work? How can a fractured limb grow back to its former strength? To understand, you first have to take a closer look at just what bones are made of and how alive they really are.

Birds have many physical features, besides wings, that work together to enable them to fly. They need lightweight, streamlined, rigid structures for flight. The four forces of flight – weight, lift, drag and thrust – affect the flight of birds.

Flying birds have: lightweight, smooth feathers – this reduces the forces of weight and drag a beak, instead of heavy, bony jaws and teeth – this reduces the force of weight an enlarged breastbone called a sternum for flight muscle attachment – this helps with the force of thrust light bones – a bird’s bones are basically hollow with air sacs and thin, tiny cross pieces to make bones stronger – this reduces the force of weight a rigid skeleton to provide firm attachments for powerful flight muscles – this helps with the force of thrust a streamlined body – this helps reduce the force of drag wings – these enable the force of lift.