Force & work

To be considered work done, there must be...
-a force acting on the object
-the object moves
-the movement of the object is in the same direction of the force

Work is measured in J(joules)
One joule is the amount of work done when a force of one newton moves an object by one metre in the direction of the force.
W=F x s
No work is done when ...
-the object does not move
-the object is perpendicular to the force
here is a video on work done:http://www.youtube.com/watch?v=IvcOYOO0Fvw&feature=related

Pressure

The S.I. unit of pressure is N/m2.
Pressure =Force /area
Pressure can be also call Pascal. 1000Pa=1kPa\
To find force,mass x gravitaional field strength = ~~~~N
To find area , length x breath = ~~~~m2
When the force decreases,the pressure decreases.
When the area increases, the pressure decreases.
One can lie on a bed of nails without getting injured as...
-the area of the bed of nails is significant
-thus the weight of the person is significantly less and the pressure would be very little.

Weight & Mass

Weight is a measurement of the gravitational force acting on an object.

Mass is the measure of the amount of matter in an object.

(e.g.weight x gravitational field strength =mass)

(50kg x 10 =500N)
Weight is usually measured with spring balances while mass is usually measured using a lever-like system using counterweights.

Forces

It is either a push or a pull exerted by one object on another.(E.g.Gravitational force, Frictional force ,Magnetic force, Elastic force, Squeezing force, Stretching force , Folding force ,Twisting force.)
S.I. Unit For force is Newton(N)
Extension/Compression Spring balance can be used to measure force.
Force can ...
-Move a stationery object
-Change the speed of a moving object
-Change the shape and size of an object
-Change the direction of an object
-Stop a moving object

Archimedes heat ray

The 2nd century AD author Lucian wrote that during the Siege of Syracuse (c. 214–212 BC), Archimedes destroyed enemy ships with fire. Centuries later, Anthemius of Tralles mentions burning-glasses as Archimedes' weapon. The device, sometimes called the "Archimedes heat ray", was used to focus sunlight onto approaching ships, causing them to catch fire.

This purported weapon has been the subject of ongoing debate about its credibility since the Renaissance. René Descartes rejected it as false, while modern researchers have attempted to recreate the effect using only the means that would have been available to Archimedes. It has been suggested that a large array of highly polished bronze or copper shields acting as mirrors could have been employed to focus sunlight onto a ship. This would have used the principle of the parabolic reflector in a manner similar to a solar furnace.

A test of the Archimedes heat ray was carried out in 1973 by the Greek scientist Ioannis Sakkas. The experiment took place at the Skaramagas naval base outside Athens. On this occasion 70 mirrors were used, each with a copper coating and a size of around five by three feet (1.5 by 1 m). The mirrors were pointed at a plywood mock-up of a Roman warship at a distance of around 160 feet (50 m). When the mirrors were focused accurately, the ship burst into flames within a few seconds. The plywood ship had a coating of tar paint, which may have aided combustion.

In October 2005 a group of students from the Massachusetts Institute of Technology carried out an experiment with 127 one-foot (30 cm) square mirror tiles, focused on a mock-up wooden ship at a range of around 100 feet (30 m). Flames broke out on a patch of the ship, but only after the sky had been cloudless and the ship had remained stationary for around ten minutes. It was concluded that the device was a feasible weapon under these conditions. The MIT group repeated the experiment for the television show MythBusters, using a wooden fishing boat in San Francisco as the target. Again some charring occurred, along with a small amount of flame. In order to catch fire, wood needs to reach its flash point, which is around 300 degrees Celsius (570 °F).

When MythBusters broadcast the result of the San Francisco experiment in January 2006, the claim was placed in the category of "busted" (or failed) because of the length of time and the ideal weather conditions required for combustion to occur. It was also pointed out that since Syracuse faces the sea towards the east, the Roman fleet would have had to attack during the morning for optimal gathering of light by the mirrors. MythBusters also pointed out that conventional weaponry, such as flaming arrows or bolts from a catapult, would have been a far easier way of setting a ship on fire at short distances.

Reference:http://en.wikipedia.org/wiki/Archimedes

Cartesian Diver

The Cartesian Diver works because the pressure you impose on the closed
system compresses the air in the bottle (water does not compress), including the
air in the diver (whether it be in a condiment packet or in a modified eye dropper).
The compression of the air makes it more dense because you are forcing the
amount of air in the bottle (and diver) into a smaller space. In order for your diver
to sink, the density of the diver needs to become greater than the density of
water. The more dense the air becomes, the further the diver will sink.

Boyle’s Law: P1V1=P2V2 is also applied in this experimaent
relationship of pressure and volume of an ideal gas (constant temperature and
quantity)

Submarines

Diving and Surfacing


­A submarine or a ship can float because the weight of water that it displaces is equal to the­ weight of the ship. This displacement of water creates an upward force called the buoyant force and acts opposite to gravity, which would pull the ship down. Unlike a ship, a submarine can control its buoyancy, thus allowing it to sink and surface at will.

To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air .When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink (negative buoyancy). A supply of compressed air is maintained aboard the submarine in air flasks for life support and for use with the ballast tanks. In addition, the submarine has movable sets of short "wings" called hydroplanes on the stern (back) that help to control the angle of the dive. The hydroplanes are angled so that water moves over the stern, which forces the stern upward; therefore, the submarine is angled downward.

To keep the submarine level at any set depth, the submarine maintains a balance of air and water in the trim tanks so that its overall density is equal to the surrounding water (neutral buoyancy). When the submarine reaches its cruising depth, the hydroplanes are leveled so that the submarine travels level through the water. Water is also forced between the bow and stern trim tanks to keep the sub level. The submarine can steer in the water by using the tail rudder to turn starboard (right) or port (left) and the hydroplanes to control the fore-aft angle of the submarine. In addition, some submarines are equipped with a retractable secondary propulsion motor that can swivel 360 degrees.

When the submarine surfaces, compressed air flows from the air flasks into the ballast tanks and the water is forced out of the submarine until its overall density is less than the surrounding water (positive buoyancy) and the submarine rises. The hydroplanes are angled so that water moves up over the stern, which forces the stern downward; therefore, the submarine is angled upward. In an emergency, the ballast tanks can be filled quickly with high-pressure air to take the submarine to the surface very rapidly.



Reference:http://science.howstuffworks.com/submarine1.htm

Fruits Float Experiment

Is it true that an orange floats in water ?Or does it sink?What happens when the orange is peeled?These homemade videos and still photos show it all!

Physics lesson 3.3.1(Density)

Density is calculated by mass per unit volume.

For example,syrofoam is less dense than wood .and wood is less dense than aluminium.

Density also affect the buoyancy of an object.

Physics lesson 3.2.1(mass)

Mass is a measure of the amount of matter in a body
The mass of the Oasis of the Sea is about 100,000 t.

The mass of an average airplane is 164t .

This is a picture showing how Bernoulli's principle works.

This allows planes to take off even when it is so heavy.

Archimedes

Archimedes may have used his principle of buoyancy to determine whether the golden crown was less dense than solid gold.The most widely known anecdote about Archimedes tells of how he invented a method for determining the volume of an object with an irregular shape. According to Vitruvius, a new crown in the shape of a laurel wreath had been made for King Hiero II, and Archimedes was asked to determine whether it was of solid gold, or whether silver had been added by a dishonest goldsmith. Archimedes had to solve the problem without damaging the crown, so he could not melt it down into a regularly shaped body in order to calculate its density. While taking a bath, he noticed that the level of the water in the tub rose as he got in, and realized that this effect could be used to determine the volume of the crown. For practical purposes water is incompressible, so the submerged crown would displace an amount of water equal to its own volume. By dividing the weight of the crown by the volume of water displaced, the density of the crown could be obtained. This density would be lower than that of gold if cheaper and less dense metals had been added. Archimedes then took to the streets naked, so excited by his discovery that he had forgotten to dress, crying "Eureka!" (Greek: "εὕρηκα!," meaning "I have found it!")

The story of the golden crown does not appear in the known works of Archimedes. Moreover, the practicality of the method it describes has been called into question, due to the extreme accuracy with which one would have to measure the water displacement .Archimedes may have instead sought a solution that applied the principle known in hydrostatics as Archimedes' Principle, which he describes in his treatise On Floating Bodies. This principle states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces. Using this principle, it would have been possible to compare the density of the golden crown to that of solid gold by balancing the crown on a scale with a gold reference sample, then immersing the apparatus in water. If the crown was less dense than gold, it would displace more water due to its larger volume, and thus experience a greater buoyant force than the reference sample. This difference in buoyancy would cause the scale to tip accordingly. Galileo considered it "probable that this method is the same that Archimedes followed, since, besides being very accurate, it is based on demonstrations found by Archimedes himself."
Reference :http://en.wikipedia.org/wiki/Archimedes#The_Golden_Crown

Physics lesson 3.1 (measuring volume)

Using a measuring cylinder as well as a Eureka can ,enables us to find the volume of irregular solids.
Measuring Cylinder
1.Fill the measuring cylinder to a certain height and record it
2.Suspen the irregular solid into the measuring cylinder
3. Record the final water level and subtract the initial level from it
4.the Equation is the volume of the irregular solid
Eureka Can
1. Fill water till the mouth of the spout
2. Suspen the irregular solid into the can
3. The amount of water displaced in the measuring cylinder is the volume of the irregular solid