Question

Explain the following natural events in terms of the laws of physics that you have learned in this class. Include figures if necessary. Answer any two.

How does a bird fly?

If you keep boiled egg inside refrigerator, it becomes difficult to take the shell off. But if you keep it warm water for a minute or so it becomes very easy. Why?

Why inside swimming pool we feel very light weight?

Mention all the laws of physics applied in these processes.

Explain the following natural events in terms of the laws of physics that you have learned in this class. Include figures if necessary. Answer any two.

How does a bird fly?

If you keep boiled egg inside refrigerator, it becomes difficult to take the shell off. But if you keep it warm water for a minute or so it becomes very easy. Why?

Why inside swimming pool we feel very light weight?

Mention all the laws of physics applied in these processes.

Answer 1

How does a bird fly?

If a bird is just gliding (or "soaring"), that is, not flapping its wings, it flies in pretty much the same way that an airplane flies. The wings push air down, so by Newton's third law the air must push them up. Partly, the push comes from the angle the wings are held at and partly from the curvature of the wings. The air travels faster above the bird's wing than it does below, and this makes the pressure lower above the wing.

Some energy source is required to overcome the unavoidable drag of pushing through the air, to lift the bird or airplane up to flying height, and to give the bird or airplane kinetic energy. Airplanes use propellers or jet engines. Birds use strong muscles in their breasts to flap their wings. In addition, bird wings are hinged, while airplane wings are riged and fixed. The bird uses its strong muscles to push its wings downwards, pushing air downwards, generating lift, and, if the wings are angled properly, also thrust. The big problem then becomes not pushing air back upwards when the bird moves its wings up for the return stroke.

This is accomplished with the hinged wings. On the down stroke, the wing is fully extended, offering its full surface area for pushing air downwards. On the upstroke, the wing folds up, presenting less area. It is a lot like rowing with oars. The oar pushes the water behind the boat on the power stroke, but must be removed from the water and, ideally, turned 90 degrees so it does not push air or water forwards on the return stroke.

Birds have a lot of adaptations for flight. Their bones are hollow and light, but strong, they have light feathers which catch the air. They can fold up their wings when they are not in use. Their lungs are extra efficient at extracting oxygen from the air (we huff and puff when running, and flying is much harder!). They eat huge amounts of high-energy food, relative to their body weight.

Why inside swimming pool we feel very light weight?

A person in water feels an upward force called buoyant force, exerted by the water acting in upward direction. This reduces the weight of the body and hence, we feel lighter in a swimming pool.