Características generales de un Amplificador Raman

principles

Try this

What is your understanding of the terms gravity, mass and weight? You do not have to write anything down, but see how the definitions given in the next section compare with your initial thoughts.

Gravity

The idea of gravity is not easy to explain. It is a force that each and every object exerts on every other object. When the object is small, the force of gravity it has is also small. Only when the object is large (and has a large mass), like planet Earth, is the force really important. Children obviously learn very early on that things fall to the ground, but it took the genius Newton to explain why this happens. Basically, on Earth, all objects are accelerated towards the centre of the planet by the Earth’s gravitational pull.

The two factors that determine ‘how much gravity acts’ are the masses of the two objects and the distance between them. You may have seen the film of astronauts on the moon. On the moon the astro- nauts weigh less than they do on Earth, why do you think this is so? Think about the mass of the moon compared to the mass of the Earth. The moon is smaller and therefore, the force of gravity is smaller. One other thing about the gravity of the Earth is that it acts equally on all objects. If you drop two objects, say a brick and a ball then they

Key point

Point of interest: Sir Isaac Newton was the first man to successfully explain the concept of gravity. His theories have underpinned many subsequent scientific discoveries, and it is in recognition of his achievements that the unit of measurement used to describe force is ‘newton’.

will fall at the same rate and hit the ground at the same time. This does not happen when you use two objects with considerably different shapes – say a feather and a brick. The fall of the feather is slowed down because it experiences air resistance, which acts against the direction of travel. Where there is no air resistance (like on the moon), the two would fall at the same rate and hit the ground simultaneously. That brings us to an important difference between mass and weight. For an object to have weight there must be a force of gravity acting upon it. Therefore, the weight of an object is defined as the force exerted upon it by the gravity of the planet.

Mass

Mass on the other hand is a measure of the matter contained by the object. The astronaut weighs less on the moon but still has the same mass. Just to complicate things further, mass is measured in kilograms whilst weight is measured in newtons, but it is import- ant to understand that mass and weight are two different things. Imagine looking at a piece of ordinary chocolate and a piece of Aero. If the two pieces were exactly the same size and you were to predict which would be the heavier you would probably go for the ordinary chocolate. This is because, for the same size, the matter in it is more closely packed. You could say the same about a piece of polystyrene and a block of wood of exactly the same volume, or any number of other materials.

When we want to measure mass we do so using grams or kilo- grams. For some, this can cause confusion, because we normally associate grams or kilograms with weight. However, the newton is used to measure force, and since weight is a downward force caused by gravity, weight is measured in newtons.

It should also be remembered that the mass of an object is constant, whereas the weight of an object can change with gravity, and this will be affected by where the object is in relation to other objects. For instance, your weight on the moon would be approximately one- sixth of your weight on Earth due to the effect of gravity, but your mass remains the same!

Weight

The weight of an object is defined as the force exerted on that object by the gravity of a planet. So, for an object to have weight

UNIT 4

Key point

Mass is given the symbol M or m and is measured in grams (g) or kilograms (kg).

Key point

Weight is given the symbol W and is measured in newtons (N).

it must experience a gravitational force, where no gravity exists then the object has no weight – but it will still have a mass. As we have already said, astronauts in space experience weightlessness because the gravitational pull of the Earth is reduced, but they still have mass.

All this talk about astronauts; fancy doing some plumbing in space? What follows is taken from a BBC news article on this very subject.

Plumbing work 390 km above the Earth

‘Astronauts Michael Lopez-Alegria and John Herrington completed their second stint outside the International Space Station (ISS) on Thursday, continuing construction work on the orbiting outpost. The two men, who went up to the platform in the Endeavour shuttle, were walking in the vacuum of space for just over six hours.

Their tasks involved plumbing work on the platform’s newest addition, a 13.5-metre-long (45 feet) hi-tech beam.

The Port One (P1) truss, as it is known, contains piping that is part of a cooling system for the ISS.

The most dramatic moment in the spacewalk came when Herrington was carried high above the space station on a robotic arm so he could guide a 270-kilogram (600 pounds) rail cart from one side of the plat- form to the other’.